If what you describe is severe enough, that's a significant disadvantage of being registered on a big instance, and using communities which are hosted on a big instance. Which in turn makes smaller instances and smaller communities more appealing.

I think it's self-regulating. The transitional period (like the current reddit exodus) is always a bit rough. Long term, things will survive which are fine for all participants.

Worst case, it's always much easier to move within the fediverse than it is to move between entirely different platforms and ecosystems. Yes, power plays and nasty circumstances are possible, but moving inside the fediverse is so much easier compared to the outside world. And being able to move is a safeguard against bad conditions.