Then there's nothing for them to extend or extinguish. If they're not able to manipulate the client and they can only control the content on their own server, what leverage does Meta have to extinguish the fediverse?
They can potentially try to make changes to the protocol, and try to leverage their users numbers to force people to accept it. I think, though, that they'll find that a lot of us are stubborn and don't like them and will not react well to that.
Which is what mastodon already does. It has extended the ActivityPub protocol in a few places. Everyone copies because you must work against Mastodon to really participate.
Change is fine when they make sense and are made by a party people aren't extremely suspicious and at times hostile to. Even sometimes if you don't fully agree with every aspect.
