Meta (or any large entity) cannot monopolize or control the fediverse. If their implementation starts drifting from established norms, they will be blocked by most instances or will just be incompatible.
Right now, many are already desperate for activity and thus hesitant to defederate. Do you actually think that you'll convince people to defederate once everyone's used to all the content they provide? "Hey guys, Meta's starting to make changes, so we're going to cut the content you're used to seeing by 99%." That's an impossible sell. Once content dependence is established, there is no turning back.
… but people forget that XMPP is still around. It never died; its just a smaller, niche network. The fediverse is already a small, niche network.
Most of us want to see the open fediverse grow into something a bit less small and a bit less niche, but that possibility will be dashed away if we put activity in Meta's hands and then let them take it away from us. Tons of people will leave platforms like Mastodon to go to Threads or otherwise have to live with most of their content being gone and no longer seeing the posts of most of whom they follow. That's lots of people who would have been sold on the fediverse but now see it as dead because of the massive activity drop. Threads coming and going takes the view of our situation from "It could grow a lot," to "It really fell off when Threads left," and the latter will make it impossible to grow again.
If we want a fediverse with the values we care about to grow, we don't need Meta. It's insane to start pretending that this is the case just because Meta is offering to control 99% of the content. Patience will help us in the long run, whereas relying on Meta to carry the fediverse will absolutely hurt us.