But Google, as a wealthy company and the controller of such a big chunk of the XMPP user base, was (maybe uniquely) in a good position to overcome these problem

But Google, as a wealthy company and the controller of such a big chunk of the XMPP user base, was (maybe uniquely) in a good position to overcome these problems. It could have established a de facto standard set of supported features, and maybe pushed out some code to help clients implement awkward bits of it.