It's had about 20 years to hit the bigtime, I think we can safely say it's not going to happen.

A mere protocol will never "hit the bigtime" without some compelling (non-protocol) reason. It's unfortunate that businesses choose to build silos time and time again, but that's where we are - it's not an issue that can be solved at the protocol level.

Should this be fixed (e.g. through regulation), XMPP is absolutely a sound choice - given, as you say, its 20 years of experience, evolution and deployment track record.

honestly, given the current adoption, growth and state of the ecosystem, I don't think it is.

In theory, you could achieve with it what Matrix achieves, but it would probably be more feasible to start from scratch - like Matrix did

That said, I see how it currently has advantages when it comes to resource consumption as well as internal deployment to centralized services.

So what is the bigtime open messaging standard?

there isn't any.

But if I would have to bet on a standard which could become it right now, then it would be Matrix.

It convinces when it comes to adoption, concept, features and growth.

> argument from authority

https://en.wikipedia.org/wiki/Argument_from_authority