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.