They're welcome to try. RSS is still going strong, and I use it daily. Spotify and ClearChannel are trying to kill it for podcasts, and that seems to be failing.

ActivityPub in the form of Mastodon will probably never be as big as Twitter was, or as Threads could become if Facebook pushes it hard enough, but I'm fine with that. That's not really a place I would like to hang out, and I like my Masto instance.