How does an instance have "total control" over users from other instances? It has no control at all. At worst it can defederate, which would just hurry along their migration to a new community that's on some other instance.
Look at Reddit: it's gone bad, and yet millions still use the site. So much so, in fact, that content on many subreddits is posted every few minutes, whereas the same communities here on kbin see hours or days between posts. That's what I mean: people are used to the solution they like, so if a community becomes "bad" enough to make me move to a different instance, it might not be bad enough for everyone else, and so I'd be stuck moving to a smaller instance while the majority of users continue using the "bad" instance. Just because I don't need to create a new account doesn't change that fact.
If I don't want to use Reddit, all of the content and users that I benefit from are still over on Reddit. No matter how much I'd like everyone to switch over to kbin, they don't think Reddit is as big of an issue as I do. Clearly. So what am I supposed to do if that happens with !startrek@startrek.website in a few years? Do I have to put up with a bad site as long as everyone else puts up with it, too? Or do I have to move to a smaller community on a different instance just so I don't have to deal with the problems of the original instance?