When they removed XMPP, they disconnected the Google users from the rest of XMPP. And since it was the largest instance, XMPP as a whole basically died. People couldn’t use XMPP to stay in touch anymore with people using Google’s thing, and vice-versa, which XMPP as a whole kind of came to rely on. I don’t know much more about XMPP, but anyways, most of my points are general and not XMPP-specific.
Also, cutting them off from the fediverse won’t have the effect you’re trying to say it will have. It’ll just be it’s own thing, just like Reddit, Twitter, Facebook, Instagram, etc are now. And the fediverse thrives in spite of them.
And please, stop assuming Facebook will play by the rules. They won’t.