Corporations don’t just sit out on new technologies, and no matter how hard you try you can’t force them to. Defederating from Meta’s new project preemptively is naive, and will not do much of anything.
Protocols are going to be adopted by corporations, whether we like it or not. SMTP, LDAP, HTTP, IP and 802.11 are all examples of that. If it ends up that meta is able to destroy the fediverse simply by joining it, that is a design flaw on OUR end. Something would then clearly need to be different in order to prevent future abuse of the protocol.
FOSS is propped up by corporations. By for profit corporations. If you want to stop those corporations from killing projects, you put safety guards up to make sure that doesn’t happen. You don’t just shut them out and put your head in the sand.
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.