Perhaps it is about providing the right hooks into the specification to not compromise the end user experience.
I think Docker is an interesting project to follow in this regard. They have shown the ability to get both startups and large players (amazon, google, microsoft) onboard by keeping the platform open. Yet they are clearly focused on providing the best experience for developer/ops interaction with cloud services. It will be interesting to see the rollout of their extension mechanisms and whether they can be a primary interface to cloud services or will become a second-teir compatibility layer. However, even in this case perhaps chat protocols could learn something here.
An interesting example is Google with SPDY. They wanted something new, built it, tested it, and then wrote the specifications (along with other actors of course). I think it is fair to say that without the SPDY experiment we wouldn't have seen HTTP/2 coming so fast. Google and the other actors had enough incentive (and actual time and money to spend) to make it go further. In the case of XMPP there's no such actor, unfortunately.
Its not broken. Standards are, ideally, distillations and applications of proven experience in a generally applicable way. You don't want standards trying to ride the bleeding edge, they want them to apply what is proven.
(OTOH, its good to have those working on the bleeding edge also opening for discussion how those experiments might be standardized while the experiments are still ongoing, and one vehicle for those discussions is draft standards or draft updates to existing standards.)
If Facebook valued compatibility with as many different products as possible, then they would keep XMPP support. The issue is not about features or XML. Most likely, it's about them wanting full control over the end-user experience: "We recommend people access Facebook Messages on the desktop via Facebook.com or Messenger.com."