Honestly, there isn't a big XMPP v. Matrix thing (or at least I wish there wasn't). A good example is on the Olm side of things, where the OMEMO guys are investigating whether there's value in using Olm as their double ratchet implementation: https://github.com/anurodhp/Monal/issues/9#issuecomment-2080... etc.

Then why, every time XMPP is mentioned, do droves of people praising Matrix and bashing XMPP show up out of nowhere?

I guess an overabundance of enthusiasm from the wider Matrix community, i guess. Speaking on behalf of the Matrix core team, we genuinely wish the XMPP community the best - for as long as Matrix & XMPP end up interoperating nicely, we're working towards the same goal of open communication. Hell, I spend most of my life atm wandering around wearing an XMPP hoodie!

If you like XMPP's philosophy of passing stanzas around, XML and XEPs, then use XMPP. If you like Matrix's philosophy of eventual-consistent synchronising conversation history, JSON and a monolithic spec, then use Matrix. Different tools for different jobs.

It's like SMTP and NNTP. You can use both of them to have conversations. They look kinda similar. You can build bridges between them. But they are totally different architectures and philosophies however and there's room in the world for both :)