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.
It’s not Meta who is going to want to federate with us, it will be a stealthy instance, for example from a videogame, so popular that everyone will want to federate with it. Including the gamers of our instance. And then this instance (which will be a facade of Meta) will implement a feature in the protocol, for the need of their videogame. Oh something really trivial…then something else. etc. And slowly the federations will diverge toward Meta.