They kinda did exactly that. GTalk became Hangouts which is no longer quite XMPP.
It's kinda sad how they ruined a superb product and replaced with something some par!
Well, not only did they change the protocol, they ruined the UI too.
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.