but long-lived public server support is almost nonexistent these days

Uhm, that is untrue, especially compared to Matrix where multiple public servers recently had to shut down because of excessive server resource use.

But yes, like in any healthy federation it is better to run your own XMPP server.

For beginner self-hosters Snikket’s guide is even less work than others, but ejabberd/Prosody are easy to setup up compared to most software. General public is generally out at needing their own server even if the system requirements for XMPP incredibly minimal & many would have access to hosting at home on the cheap with dynamic DNS & basically anything with a processor + a Linux distro.

Not sure what the modern feature support you are talking about tho. Some clients already have stickers, reactions, threading… but the ‘X’ is for ‘extensible’ so it is all meant to be optional on purpose so it is easier to implement clients & democracy wins on features that clients decide are worthwhile to uptake (at least now that Google is out of the picture dictating too much)--& you have community-ran compliance suites for server features like the one for Conversations. Having used a couple of Matrix clients that aren’t Element, the fragmentation of client feature support is literally just as bad--except there is a lot less maturity due to age.