I realized that my previous post didn't embed the rebuttal link I was meaning to send, so please give it a look, sorry for that.

But decentralization also means it takes a long time, sometimes a very long time, for the that particular product’s benefits to become known and widespread.

This is true: one can't possibly come up with a new chat product within few months would they want it to be an internet standard (which XMPP is) and having diverse parties implementing its components (which XMPP has). My question to that is, do we really need new chat products every other month? I personally consider that instant messaging was a solved problem 25 years ago when AIM/MSN & al. were ubiquitous and used by everyone and their aunt. Arguably, current generation's messengers pack less features, not more, than those they precede, and that's a trend we also observe within XMPP: old specs defining how to game together, share tunes, share whiteboards, … are slowly fading into obsolescence.

Nowadays XMPP has all the relevant features one would expect, in spec and implementation. There was a time when XMPP had problems with mobile use, not because it wasn't adequate (it was successfully used over extremely low bandwidth before), but because Google and Apple had decided that they would silently kill clients and a new protocol had to be figured-out for that event. That's perhaps a "once in a decade" evolution, which happened at a time the XMPP ecosystem wasn't as vibrant as it is today.

I believe that’s what happened to XMPP, IRC and so on.

IMO what happened has nothing to do with "features" nor "moving with the times", what happened was a lot of venture capital money to answer every tech giant building their own walled-garden messaging platform. Again, all current "modern" messengers can do pretty much the same thing, look the same, and came-up roughly at the same time (and yes, this applies to Telegram, too).

This is a danger that applies to Mastodon and Lemmy/Kbin too […] become obscure

This is not a danger, this is inevitable. Don't expect Lemmy/Mastodon (and the Fediverse in general) to become mainstream this generation: the internet doesn't work on merit, people don't spontaneously lean towards what's "best", even for themselves. Only tech enthusiasts do, and as it happens, they have a negligible political and societal impact compared to the (tech) majors. IMO no amount of persuasion by the geeks will change that. What I believe matters is, on one side, to define and standardize future-proof protocols and have them audited for security (XMPP is uniquely positioned here), and on the other, to lobby politicians so they make use of the existing legal framework (forbidding anti-competitive practices and monopolies, mainly) to level the playing field and compel the majors to become interoperable using said protocols. Mozilla may play a role in that, but what's going on with the EU and the Digital Markets Act is worthy to keep a close eye on.

Edit: typos and rephrasing