I guess it is impossible to say what would have happened if Google never used XMPP. To me it mostly looks like google joined XMPP and made it way bigger than it was before and eventually left it again, making it small again. But is it worse than before Google even joined?
Maybe, but can we say for sure?
Maybe the lesson is not "don't let the big corporate players in", but rather "make sure the development of the underlying protocol itself is done in an open way".
If Google/Meta adds proprietary extensions, just don't add them to the main protocol. If they leave the protocol again or changed their implementation in a way that is largely incompatible with the open version, nothing is lost than what they brought in initially.
Doesn't that make sense?