XMPP is mature. I think that the downfall is the complexity coming from having almost everything defined as an optional feature on a protocol level, and at a very fine granularity at that. So an implementation has to cope with situations where any other party it wants to communicate with may have an oddball collection of supported features. Plus, some server operators added their own peculiarities on top, like wanting to federate with some servers and not others and so on. The user facing result was a user experience with seemingly random inconsistencies. For example, user A can send a message to user B while user B is offline, but not to user C because C's server doesn't support offline message storage.