Have you used XMPP recently and ran into the issue of non-obscure servers, clients, or self-hostable implementations using different extensions or not supporting them? (I actually haven’t experienced this even on the obscure ones, but can’t confirm for all of them). Please do not make that accusation, because that I’d really not what happens in reality.

it’s hardly the standard

Why not when… It literally is? And all major implementations follow it? That is by definition a standard.

and we’re not really talking about plain XMPP then anymore.

Why not? “extensible” is in the name. It is meant to be extended. The protocol is being used exactly as planned and intended.