I’ve read quotes by Greg stating that Mata Nui being a giant robot with his head under the island had been planned from the beginning. As it is indeed likely, considering the names of certain places on the island (Mount Ihu = “nose” and the volcano Mangai = “mouth” in Maori). Greg also stated that if you remove this out of your head canon, it’s like removing the bottom card in a house of cards, causing the whole thing to collapse in on itself.
Yet, how then was an entire generation of kids able to enjoy Bionicle for almost a decade without knowing this until shortly before the end? How were they drawn into the story without knowing this if the entire worldbuilding supposedly relied so much on this secret? And if the commercial success of Bionicle long before the big reveal can be accepted as proof that this piece is in fact not as integral to the worldbuilding as the inventors might believe it is - maybe that big-robot reveal did more harm than good to the franchise in the end?
I’ve been with Bionicle since -2 years before its beginning. And by -2 years, I mean I got into Lego Technic through the Slizers, stuck with it during the RoboRiders, and then came Bionicle. It would have been just another short robot series for me if it hadn’t been for the mini CDs included with the Toa Mata that explained the story so vividly. This made me realise this was not just some random series of toys; there was actually a mythology behind it. I stopped collecting the toys after the Visorak, i.e., the Horidka were the first ones I didn’t get. Yet, I continued to follow the new releases, watched the 3rd and 4th movie, and read the comics all the way up to the last one that got released that I am aware of, which took place shortly before The Legend Reborn.
In that sense, I do believe that Bionicle is kind-of a miniature Star Wars for the Millennium-Slizer generation. 
Not just because of the structure of the storytelling (main plot, prequels, return to after the main plot), but aso because there are SciFi elements (in this case, robots), but the thing that makes it memorable is not the mere action entertainment in terms of combat and visual effects, but the mythological / “spiritual” connotation to the story. In Star Wars, everything revolves around the Force; in Bionicle, the entire plot from the beginning revolves around re-awakening Mata Nui.
But the big robot reveal with the face under the island, especially paired with the claim that this was planned from the beginning, implies that it has always been clear that awakening Mata Nui would lead to the destruction of the eponymous very island the initial story is set on - the very island the characters are trying to save by awakening Mata Nui. And that this destruction, albeit non-intentional on the Matoran’s and Toa’s part, was indeed intentional on the authors’ part.
That’s as if the rebels in Star Wars had conquered the Death Star instead of destroying it, but then it had accidentally blown up Tatooine.
Even the deconstruction of Star Wars and textbook example of “subverting viewer expectations” that is The Last Jedi did not have the b*lls to go that far. But Bionicle did. The Big Robot rising and destroying the island of Mata Nui, that was the Last Jedi moment of Bionicle (long before The Last Jedi even existed). The fact that Makuta had taken over the robot was just adding insult to injury.
Although Makuta winning was of course the first thing I noticed when I read the very last official comic depicting this event; I hadn’t realised at that point that the island had been destroyed, which was probably for the bettter - ignorance is bliss. I only finally understood it once I recently got back into Bionicle and started brushing up on the lore - thanks to all the fan-made video games that are being developed now. And let’s say I believe there is a reason that all of those fan games revolve around and are even limited to the Toa Mata era, the very beginning of the 2001 storyline.
But if you accept the eventual outcome of the canon story for yourself, if you play any of these fan games, all the while you would have in the back of your head that what you’re doing is ultimately pointless. The island of Mata Nui can’t be saved, nor can Voya Nui. And by pursuing Mata Nui’s awakening, you’re even actively working towards their destruction.
Around the same time that the robot-twist was revealed (2009), one of my favourite role-playing video games came out: Divinity II: Ego Draconis (later expanded with “Flames of Vengeance” and then renamed to "The Dragon Knight Saga). I personally didn’t discover this game until 2012. But it has a similar twist at the end of the main game, which also does not only render everything you’ve been doing so far pointless, but you’ve actively made things worse. And that would have been the actual ending if they hadn’t made the “Flames of Vengeance” expansion later on, which at least gives you the chance to rectify that mistake. However, even in the expansion you still don’t accomplish the main goal you were actually pursuing in the main game. This expansion is thus basically like “The Legend Reborn” for Bionicle: That movie set up a plot where the mistake of awakening Mata Nui’s body with Teridax’s consciousness could be rectified - but it didn’t end with the vindication of actually seeing that happening.
Sometimes I think there should be a Power Metal band dressing up as Toa on stage (I mean, “Legend of Mata Nui” already shows Toa dancing as part of the Takara, so it’s not too far fetched). Because much like Power Metal, Bionicle always had the issue of not quite knowing whether it wanted to be Fantasy (with the environment of wilderness on the island of Mata Nui, and with all the mythology and superpowers) or Science Fiction (given that all the characters are robots). But for the longest time, it bridged the divide between Fantasy and Science Fiction quite well, much like Star Wars was essentially planned as a SciFi Fairytale (it even starts with “A long time ago…”, after all - or, in our case: “In a time before time…” 
).
However, the giant-robot reveal at the end placed the franchise entirely in Science Fiction, and in doing so basically made all the mythological explanations the Turaga and others had come up with nothing but false information in hindsight.
- “awakening Mata Nui” is just a fancy term for starting a program designed to launch the robot back off Aqua Magna in case of an apocalyptic event
- the Toa are essentially just an anti-virus program to protect the robot from the virus the Makuta had infected it with
- Most importantly, there isn’t any actual necessity to wake up Mata Nui to get him away from the planet, because there is no imminent threat (in contrast to the meteor that hit the Slizer planet
). The Turaga just think that’s what the Toa have to do.
The entire Bionicle mythology is now just an example of a failed belief system: Something that put ideas in the Matoran’s and Toa’s heads that were false descriptions of reality, arisen from a mere lack of knowledge, and pursuing the awakening of the Great Spirit almost would have wiped out the entire civilsation at several occasions.
Just imagine the Toa Mata would have succeeded at awakening Mata Nui right after their first confrontation with Makuta - nobody knew about Metru Nui yet at this point, so the island would have been destroyed before anyone could have been evacuated.
And in Mask of Light, it’s only fortunate coincidence that Takanuva had the idea of gathering all the Matoran in Makuta’s lair - and that Makuta made the decision to prevent them from going back to the surface. Again, if they had actually awakened Mata Nui right then and there, as the movie implies it happens, then anyone who was still remaining at the surface would have been “Mata Nui bone”, as Jaller would put it.
Also, the inspirational messaging of Mask of Light is sabotaged in hindsight: When Hahli gathers everyone to lead them down into Makuta’s lair, following Takanuva, she describes the island of Mata Nui as a paradise and says how much she loves her home. She ends her speech with the call to awaken the Great Spirit today. In doing so, right after she states how much she loves her home island, she unknowingly calls for the destruction of that very island in the next sentence.
The idea of Mata Nui being a giant space robot with biomechanical inhabitants to do maintenance work (and the creation of a surface with vegetation on his face as a type of "terraforming) does of course raise interesting questions: What do you do when your old planet is doomed by some cosmic disaster (as Earth will be eventually, too)? What do you do if the new home world you colonise is facing such a threat later, as well (–> awaken the robot so that he can leave the planet)? What are the dangers of artifical intelligence, and in particular: What happens when artifical intelligence develops their own system of values and beliefs?
Those are certainly interesting questions, but maybe they could have been answered much better in a different franchise. Not by putting the mythological Fantasy parts and the SciFi parts into the same story.
As much as I appreciate that the complete Bionicle storyline now shows the potential dangers of the Matoran belief system that was used as a quick and easy substitute for an actual, “scientific” explanation: The “truthful” explanation behind all the initial mythology completely disenchanted it.
The designers first created a fictional mythology that enticed a generation of kids into this fictional world, only to then completely destroy that mythology at the end by revealing the actual truth and inspiration for those myths.
Maybe it was cleverly timed in such a way that the kids who got into Bionicle at 6-8 years old would be teenagers or even adults when the storyline ended with the big reveal. Maybe the authors thought “now they can handle such a big hit to the mythology they cherished as a kid”.
If that was the authors’ way to teach us something about the power of myths first-hand - enticing us into it as kids, then having to face reality as adults - I gotta say: That was pretty darn genius. Almost a psychopathic level of genius. The question is: Was it worth it?
Kids didn’t play with Bionicles to be taught something. We didn’t want it to be the Matrix, where you have to choose between the Ga-Capsule and the Ta-Capsule, to see how deep into the robot’s body the rabbit hole really goes. We wanted it to be our miniature Star Wars: Primarily escapist. Yes, with some philosophic messaging, why not. Star Wars’ “fear leads to hate leads to suffering” vs. “Unity, Duty, Destiny”, that’s all fine. We did not ask for a bait-and-switch game with a sudden change of the genre of fiction at the end, replacing a simple but inspiring Fantasy story with an overly complex and frankly completely absurd SciFi story.
Don’t get me wrong, I do like stories where supernatural and magical explanations are discarded for much more mundane, rational, technological / “scientific” explanations. That can incentivise kids to use their own rational thinking (“Sapere aude!”, as Immanuel Kant said). But that obviously doesn’t work if the technological explanation is even more abstruse than the mythological explanation has ever been.
If I now go to any Bionicle wiki page to look up literally anything, all the important actions in the grand scheme of the storyline were undertaken by characters and entities that never existed as toys in the first place.
None of the places we were made to care about over the years - the island of Mata Nui, the city of Metru Nui, Voya Nui - exist anymore, while the new world the survivors live in at the end, Spherus Magna, is something we know next to nothing about.
The original protagonists that drew us into the story in 2001, the Toa Mata, are relegated to being a footnote in the history of the cosmos as Mata Nui, formerly just an eternal off-stage character in the background, became the new main character in the last act of the story. Suddenly making the formerly most powerful but completely absent character in the setting the central agent of the final lap of the story - the term “Deus ex machina” has rarely been more fitting.
Heck, even the antagonist’s motivation has changed repeatedly: Makuta is not only no longer named Makuta, he’s also no longer Mata Nui’s brother, but one of his creations, and he’s no longer acting out of jealousy towards his “brother”, but out of sheer and “boring” lust for power, thereby eliminating the original parallel to the story of Cain and Abel.
Also, I’m pretty sure that on the original Toa Mata mini CDs, the story was told in such a way that the chronological order was “first there was the island, then Mata Nui arrived, and the Matoran (then Tohunga) named it after him in his honour, and then Makuta arrived and put Mata Nui to sleep”; this can’t be reconciled with the “actual” explanation either, because the island of Mata Nui can’t even exist before the Great Spirit robot falls asleep due to Makuta’s actions and crashes into the planet. But I guess that was just another thing the Turaga misremembered, another mythological lie they told the Matoran?
Whether the official ending would have been Teridax’s triumph by taking over the robot’s body, or the definite ending of Mata Nui returning and destroying Teridax for good, both feel like a tragedy to me.
I don’t care about Spherus Magna being one again. All things considered, it just had two moons crash into it, and thus should be uninhabitable for at least 100,000 years. (That’s what would happen to Mars, btw, if Mata Nui were somehow able to give it an atmosphere, causing friction with its too moons that would consequently slow down and fall onto Mars.)
I don’t care about the inhabitants of Bara Magna - I’ve only briefly gotten to know them in The Legend Reborn, the worst of the four Bionicle movies by far, as a cast of largely cringe-worthy characters, including Bionicle’s very own Jar-Jar Binks, because that’s something the universe totally needed. 
I don’t care about the Elemental Lords or the Great Beings - they never existed at toys, they were never introduced to me as individuals I could relate to. I care about the characters we were actually made to follow, and they’ve been robbed of any possible place they could have called their home.
In the end, I can’t come to any other conclusion than that the giant space robot Mata Nui should have remained asleep. I’ve even heard something along those lines was one of the original planned endings: Mata Nui still rises, destroys the island, gets expelled from his body by Teridax, then returns, defeats him, lies down again because his purpose is to be asleep so that his inhabitants can live on the surface. They rebuild the island of Mata Nui, and we’re essentially exactly where we started (Prince of Persia / Butterfly Effect storytelling), just with a long and contrived path in between to ultimately defeat the Makuta.
So the best outcome for the beloved island paradise of Mata Nui would indeed have been if the robot had never awakened. 2003 Makuta was right: Awake, he suffers. Since any way of waking him up destroys the island, the only way I can keep enjoying those new fan games based on the 2001 storyline without constantly thinking “Tahu, stop pursuing this goal, you’re being stupid; Vakama, stop putting these ideas into his head”, the only way for me to keep thinking that awakening Mata Nui was actually a good thing, is if “awakening” somehow only means “uploading his consciousness from the doomed and buried robot body into a cloud, some satellite somewhere in the orbit of Aqua Magna, from where he can communicate with his people again”. Preferrably without the Matoran, Turaga, or Toa even realising that there is a technological thing going on behind the mythological thing they’re doing. At least that’s my best solution if you don’t want to give up the premise of “there’s a giant robot lying dormant under the island”.
Alternatively, if you scrap the giant-robot explanation altogether, the only thing you lose, in my book, is the hidden meaning of the terms “Ihu” and “Mangai”. That seems like an acceptable loss to me .
There is a reason why Star Wars fans also love the original three movies the most.
The Bionicle equivalent to Episodes IV - VI are the first three years: The struggles against the Rahi, the Bohrok, and the Rahkshi.
The following prequels (Star Wars Episode I - III / the Toa Metru and Hordika arc) are nice add-ons to the world building.
Episode VII and the Voya Nui / Toa Inika plotline was a re-hashing of a previous plotline (“The Force Awakens” = “A New Hope” reloaded; the Mask of Life = Mask of Light reloaded).
Episode VIII took a big steaming dump on everything that had made Star Wars inspiring (–>Mata Nui robot rises, while being controlled by Makuta, and destroys the island).
And then, Episode IX was a kneejerk attempt at fixing the fallout of Episode VIII (–>Mata Nui returns and finds some contrived way to defeat Makuta for good and quickly force the story towards a happy ending because we don’t want it to end on a somber note)
Therefore, as far as I’m concerned, the story ended with the awakening of the Great Spirit - you know, as a spirit, not by the resurrection of his physical form - at the end of the movie “Mask of Light”.
I’ll try to address each point separately, as I view it:
In turn, I can of course see how somebody who got into the storyline later, e.g., with the Toa Inika, got less attached to the tribal themes. As I just heard again in one of the TTV videos on YouTube how Lego deliberately cut back on the spiritual themes in later years to avoid accusations of religious themes in Bionicle.
!
…
I’ve always perceived the three ellipses more as ovals than as round, planet-like shapes. Even when I saw the depiction of the symbol in the stars in the opening of “The Legend Reborn”, to me it looked more like “ah well, a new take on the logo, that’s cute” instead of “finally an explanation for why it looks this way”. I could just as wel ask myself why the Matoran writing system looks the way it does. Or why it changed from circular outlines to polygons somewhere down the line.
Both the mythological and the technological explanation are supernatural, of course, but the mythological explanation is not as needlessly complex as the technological one. Ockham’s razor applied.