XMPP was never popular to begin with, because it's a messaging service that relies on the people close to you using it, which was rare before Google Talk integrated. Corporate run apps brought direct and indirect usage, you can't argue this is an overall loss when they pulled away from XMPP, at worst it's the same as if they never integrated. The same is true for ActivityPub, whether everyone defederates or blocks Meta instances now or they stop supporting ActivityPub later makes no tangible difference.

I went to university in the 2000s at a smallish German Technical University. Rarely anyone used jabber. What literally everybody in the early 2000s was using was ICQ. Every dorm had ethernet, everybody had a PC and everybody had ICQ running 24/7. The ones not living on campus were peer pressured into getting DSL (which was still uncommon elsewhere).

Then came Facebook, and suddenly all those ICQ contacts were gone. Still, rarely anyone used jabber, only those who didn’t like Facebook. I didn’t know a single person who was on Google Talk.

Then came Android, iOS and Whatsapp, and that’s what „killed“ XMPP, because XMPP was so not ready for mobile networks.