They kinda did exactly that. GTalk became Hangouts which is no longer quite XMPP.
Well, not only did they change the protocol, they ruined the UI too.
It's kinda sad how they ruined a superb product and replaced with something some par!
X in XMPP stands for eXtensible. They could simply augment XMPP to match their needs better - and guess what, it's exactly what happened in last years. Now XMPP is a good, modern, battery- and UX-friendly IM protocol, with some clients even E2E encrypted by default.