I doubt any platform uses vanilla XMPP, that wouldn't be sane, but like using XML, XMPP is a relic of its era. Whose to say what super-critical "how can you not have this as standard!?!" feature won't come up in the next 5 years that Matrix devs hadn't even known of in 2018. That doesn't mean we should flip over the table and start again every now and again and hope the users will follow, this is why extensibility is an important part of the recipe and make ways to ensure the standards of the day are always implemented before being considered ready for prime time.