I think it's about keeping the userbase on ActivityPub as much as possible. When meta will start doing ActivityPub and probably change it, everyone will need to follow because Meta will own all the userbase and "subreddit".

At some point, they will decide to drop ActivityPub because it's not good enough for what they want to do. Just like what Google did to XMPP. And maybe Google was right about XMPP, I don't know.

Another reason is what you are saying, a personnal battle against Meta and big corp.

At the end of the day, will anything the fediverse Admin do will matter ? Only time will tell I guess.

One key difference with HTTP is just like TCP. Everyone uses it so it's much harder to just change it and fuck everyone else. ActivityPub is an easier target for this strategy.