"Awesome! Yay! This movie is the start to something new! These new movies are gonna be awesome!" says everybody...except the Bayverse fans.
Going back to the Spider-Man point, I felt similar feelings of disappointment, sadness, and even mild frustration when The Amazing Spider-Man series was cancelled after the second movie left so many loose ends. Even to this day, I still wish that they would tie up those loose ends. But since Spider-Man came to the MCU, things have been great. Homecoming and Far From Home were even better than TASM, and we got to see Spider-Man in several crossover movies (plus, there's a rumor he's gonna be in a Captain Marvel team-up movie).
So for all I know, this new reboot that's supposedly coming in 2021 will be a similar case. I have never said I think it'll be bad. In fact, chances are, I will see it if the rumors are true. And despite everything I've said, Bumblebee is objectively a good movie. All this prequel-reboot controversy has somewhat tarnished my vision of it, but it still has its good points. And it has enough that it can still work as a prequel (which is probably what the studio had in mind when they were retooling it).
But back to the original point. To promise us a prequel, deliver that prequel, and then go back and claim that it's a reboot instead, feels like a big middle finger to us Bayverse fans. Sure, there's the whole deal about wanting to turn people on to more Transformers movies, but there are still a lot of Bayverse fans out there who are alienated by this. And just because people will be okay with this reboot going forward, it doesn't mean there was never any controversy in the first place. The official Transformers wiki still lists Bumblebee as a prequel to the Bayverse (don't believe me? https://tfwiki.net/wiki/Bumblebee_(film) ). Really, they've dug their own hole.
Sure, there are these reasons for this issue supposedly being okay. But making claims about your work after you've released it, saying that this or that thing is actually very different than it is, is still wrong. I mean, look at J. K. Rowling. In recent years, she's been going around claiming that this girl was actually black, and that guy was homosexual the whole time, and these guys were actually not allied with the D. A, or whatever. She's only doing that because she wants to stay relevant-and when you think about it, that's kind of what Transformers is doing right now. They want to stay relevant, so they're openly contradicting things they have said before.
The truth is, at the end of the day, a reboot is still the best way for the franchise to get back on track after TLK was a failure in the box office. That story is unresolved, but the cliffhanger existed only in the form of a mid-credits scene. If it wasn't for that, then the movie would've felt more like the end of its own story. But of course, the prequel Bumblebee was still being made. And as this discussion should prove, changing it to a reboot at the last minute doesn't go over well with everyone. So these are the possibilities that wouldn't cause any problems:
1.) Don't do anything, leave Bumblebee as a prequel, and just stop making Transformers movies altogether.
2.) Cancel Bumblebee altogether (this would be the worst option, because by the time TLK came out, they had already started filming it, and had spent a bunch of money on it).
3.) Keep Bumblebee as a prequel, and then say something like "Oh, you like this movie? Well, good news. In a few years, we're gonna reboot this entire franchise, with the same director that made this movie you liked!"
4.) This one is weird, but it might work better than any of the above points. State that Bumblebee is both a prequel and a reboot at the same time. I know that doesn't make a lot of sense, so hear me out. I'm only really in the Bayverse fandom, but from what I've read, every Transformers continuity (G1, Animated, Prime, etc.) exists within a multiverse, sort of like Into The Spider-Verse. This means that each reality has its own timeline. So perhaps Bumblebee could be a situation where the Bayverse's timeline and the reboot's timeline spliced together. In other words, the events of Bumblebee happened in both timelines, but at the end of the movie, the timelines branched off again. This is why there are elements of both timelines in the movie.
I think the last one is the one that would work best. It would give us Bayverse fans the prequel we want, while also letting the franchise reboot itself.