> _why_ communication has moved away from email
Has it, though? I don't think we are any step closer now to dropping email than we were 25 years ago.
> or why XMPP/IRC never really caught on despite being vastly older than these app competitors
Could be the lack of marketing (because there isn't a corporation or venture capital money behind benefiting from exponential user base growth) and critical mass/peer-pressure as a result
> making sweeping changes to the protocol/client in the name of UX becomes intractable.
This starts to be dated, but is a counter-example: https://gultsch.de/objection.html
XMPP keeps evolving, and probably faster, and with better adoption than something centralized like Skype for instance.
Moreover, we've arguably less feature-packed messengers now than we had two decades ago.
> It is _very_ frustrating that the internet has had messenger fragmentation since the beginning of time, but why can Matrix solve this problem if XMPP cannot? Signal just works.
The irony. Signal contributes to fragmentation. We seem to pretend that it's a protocol/technical issue, but did you know that WhatsApp runs on XMPP, so did Google Talk, and Facebook Messenger early on? All those silos could open federation and let messages be exchanged across services, but chose not to do so (including Signal), because they have more to gain in keeping you hostage.