XMPP had all that, but there was no single application that implemented all of that.

You seem to underestimate a lot the messaging landscape of back then :) It wasn’t a “XMPP thing” to implement those for laughs and giggles, it was the qualifying feature set of every popular app then, and XMPP clients like Gajim, Psi, Pidgin, … were competing against multi-protocol clients like Adium, Trillian, Telepathy, … and 3rd party ones (like aMSN, KMess, …) to implement the features of MSN, Yahoo, ICQ, … Yup, it was a zoo, but it was not XMPP’s. The only point of shame I see here is that we lost pretty much all the fun and innovation when everything became “mobile-first”.

I understand your defense of open standards and I’d love if the bazaar model could’ve worked for XMPP. Unfortunately, it didn’t. It is taking a Cathedral to come up and implement something that is at very least workable in all major platforms and still open for those that want to deviate from the main effort.

I don’t think this analogy is relevant there, and I certainly don’t think that the multitude of XEPs I listed are evidence of XMPP being akin to a bazaar: the XMPP protocol is very lean and coherent, and kept that way by the XSF. Those XEPs were meant to document and standardize the practices of the time.

But I can have my wife and my parents install Element on their phones (android or iOS) and be talking with them in less than 10 minutes, but I can not do the same with XMPP without having to accept a huge amount of compromises.

I have no idea what compromises you are talking about. I have SO, parents and many more, of all ages, abilities and systems having joined XMPP, with no difficulty whatsoever. The experience probably even is better here than on Matrix because not only can you generate invite links/qrcodes that contain a bootstrapped contacts list, there is also the option to discover contacts via mobile phone numbers (e.g. from quicksy.im, choegram) that comes handy in some contexts.