Don’t know exactly, but it’s what happened with Google and XMPP.
Same thing can happen here.
But I’m imagining that Big Tech starts to post from their own server with content that is interesting to the the masses, rather than us nerd, then they market to all the other servers to get a bunch of sign ups (maybe with exclusive content or features, idk), then federate with their own other websites and servers using their own proprietary ActivityPub, then bam OG ActivityPub never takes off, withers and become even fewer nerds like me, and and a bunch of bots with little content.