Making it so people can't see that content will just make the fediverse become more centralized, because people will just go to the bigger instances that will allow for them to see that content, or just go sign up for threads.
Perhaps I'm misunderstanding something, but it sounds to me like you're still saying "if Meta is providing the content that people want and it's not available elsewhere then people will go to Meta for it."
And that seems fine by me, in that case they're competing by providing the content that people want. However, not everyone wants the same thing. I have no interest in "following" politicians or celebrities. A lot of the sorts of conversations I'm interested in do not benefit from having a huge number of people in them. So even if Meta is some kind of juggernaut there's going to be people (like me) who don't want to participate in a juggernaut. The smaller defederated instances will still be attractive.