While the various xep might be a confounding factor - the biggest issue I see today with xmmp is that end to end encryption and perfect forward secrecy is optional and bolted on.

I think Facebook and Google's embrace-extend-extinguish (both fb messenger/chat and gtalk used xmpp - neither allowed federation - and created artificial silos) was probably the worst thing for open standard chat.

Gmail isn't quite big enough to have been able to fracture email yet - but not for lack of trying (years of poor imap support, strange handling of threads/top-posting etc).