@cyclohexane I can only speak from my personal experience having hosted both XMPP and Matrix for friends/family before.
Ran XMPP (eJabberd) for round about 10 years and it never really was a trivial process, neither for me as admin nor for my friends/family with regards to parcipating.
Basically, back then, I had to manually extend eJabberd with a bunch of XEPs (namely push notifications, message carbons and message archive) to increase the useability and user convenience to even stand a chance getting people on board and able to use the system. The client ecosystem was not quite there yet either - Conversations for instance had just come around to shaping up for android, Gajim for cross-platform was pretty fine though.
Let's not talk about E2E encryption either: GPG - not a chance, OMEMO was just coming around as well and was not yet very reliable.
Matrix on the other hand was quite the breakthrough for me as an admin with regards to user acceptance. I do believe that a big part of that comes from the concerted effort to have a unified client (Element) available on any platform - web, fat clilent, mobile client.
By now there's also a ton of cross chat platform bridges which also greatly serves as a "selling point" towards users. And most imporantly, again in my humble opinion, the required technical knowledge barrier for users is just not comparable to XMPP.
Don't get me wrong, I've learned so much as an admin setting up and hosting XMPP and for a short while I even had a PoC going at work to try and advocate the protocol, but in the end Matrix feels like a worthy successor to me.
It allows me to convince "normal users" to use a federated, self-hosted and free chat platform reliably - and that's what mostly matters to me :wink: