Technology

with no user benefits for this extra complexity,

This is not true. One of the benefits I've enjoyed greatly is being able to point a group of non-techies at a URL and have them up and running, with fully encrypted channels, in less than 5 minutes. Another benefit is chat history across multiple devices. There are more.

with only a single entity [...] being in charge

Also not true. It's an open protocol with a well-defined process for contributions. The original authors certainly have influence, but I haven't seen any gatekeeping there.

of developing both the client and the server.

There is no single client or server. There are multiple clients and servers (with varying levels of resource usage) from multiple developers, and the available options continue to expand.

I'll grant that XMPP is a simpler protocol, especially in its minimal form without the various XEPs that are needed to even approach a comparison, but it also doesn't accomplish as much. That's not a "red flag" for Matrix. Also, this article is specifically about a new protocol.

I am happy to self-host XMPP accounts for a few hundred people, family and friends and to no longer have to do it with Matrix.

Yes, your case is an example of a small community with an informed and involved admin, which I pointed out in my original comment. XMPP can make sense there (I've done it too) but it's a niche within a niche. It doesn't address the problems that Matrix solves.