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.