Well, if met~~h~~a decides to interop, activity pub can launch some propaganda to make users "just switch", I guess. I mean it's somewhat different from the situation with xmpp and google as most sane ppl already know fb's crap isn't good for your data... So maybe smth like "remember that cool shiny thingy people were leaving xitter for? Guess what, now you can talk there with your grandma without subjecting yourself to fb's shady practices" could work given interop works good enough.

P.S. that specification under a EULA actually sounds like a good idea if you put it a bit differently: whatever implements this specification should be published under one of $insert_a_set_of_licenses. Then whatever proprietary garbage creator that decides to join will be forced to do this via bridges, and others can tell them to fix their crap 😁