What if the masks stop the villagers from aging? They provide them with no actual energy, but instead keep their life force from leaking out of them, which keeps them how they are for however long they have on the mask. More powerful masks provide actual energy, and this energy is increased the stronger the mask is. However, the more power in the mask, the harder the shock is when they’re removed. The villagers only have strong enough bodies to support the life preserving masks, but when they lose those, skull spiders go to feed on the leaking energy that the mask kept in, until the host is powerless, allowing the spiders to take full control and use its own energy to fuel the body. The skellies have these masks as well, and use them to keep in the dark energy Grinder gave them. Skull slicer on the other hand, is an example of what happens when a skull spider gets full control.
Toa however, have the golden masks, but when those are removed, they become weaker and weaker due to the sudden drop in power input. The skeletons draining these masks quickens this process. But that’s not it. No, when they were fighting Kulta and he knocked their masks off, they were all very, very weak. That’s because their bodies were used to taking in the golden mask’s power and the drop was absolutely crippling. Also, if you’ll remember back to when they first got their masks, the narrator said they were covered in their element. That was simply the shock of the new amount of power, and their bodies getting used to it, which also allowed for a massive boost in the power they could use. Because they already had their elemental power flowing out of them.
Moving onto the mask makers, we saw what happened with Ekimu, he lost his mask and BAM he was out cold for a loooong time. Not fun stuff. The only reason he survived was because the burial mask kept the small amount of energy inside of him, and when the Toa hit him with elemental powers, it gave him a bit of a recharge, making him awaken. The MoUP is MUCH more powerful than the brother’s legendary masks, so when that was put on, the raw power mutated the host due to the power output. This power output was so great, not only does it leak out of the mask (the flames around it), but when it’s knocked off, it doesn’t stop feeding power and that power destroys everything.
Now I'm not done, there's actually a way this theory could tie into 2016. The golden uniter masks (which were confirmed to be necessary for uniting) are used as a bit of a transfer point, that adds the creature's elemental power to the toa's. With the normal mask, you just have a creature on your back. and that's kinda useless.