Here's a specific complaint about XMPP, and possible explanation why nobody uses it any more. (I worked on a large-scale XMPP implementation back in the day.)

Presence. That's the colored dots indicating "is somebody online or not". The traffic needed to maintain presence scales by N^2, and in any large-scale implementation, the traffic to maintain presence data completely dominates anything useful.

Not to mention that for the past 15 years or so (ever since everybody has a connected cell-phone all the time) the whole idea of presence (am I online or not?) is either meaningless or just badly modeled.

So the result is a protocol which spends tons of bandwidth and battery maintaining metadata that is functionally useless. That's why the real world has run away from it as fast as possible.

>The traffic needed to maintain presence scales by N^2

Only true if we assume the average number of contacts scales linearly with the total number of users, right? But then, we could also assume that the average number of messages that a given user sends scales linearly with the total number of users, in which case the amount of traffic needed to transmit messages also scales by N^2.

A couple of easy fixes: cap the number of contacts at some large constant (as many services do), or just disable presence information altogether in your implementation. I'm skeptical that this played a major role in XMPP's lack of popularity, especially because e.g. WhatsApp and Facebook Messenger have presence information, and are still popular.