I read this as a trivial 'truth' that I've internalized myself as well; but the way you worded it... makes me naively ask:
doesn't it speak to the limitations or choices of XMPP itself then, not the idea of a common protocol, if businesses can't reliably extend the protocol (only custom fork, and somehow barring eventual upstream contrib, whatever)?
I can think of an extensible protocol paradigm, with 'extensions' (really just libraries, packages, like those we fetch with `apt` or `npm` and optionally push to clients as well). Down to first principles, that's how we structured UNIX/Linux, most modern programming languages, the web itself (well, at least the Js and now wasm part of it).
(Thinking out loud...)
