I think the most valid criticism of Matrix for interoperability is that it was promoted as an interoperability standard but it was made incompatible with everything existing at the time. XMPP was based on XML for example, but Matrix is based on json. So by introducing yet another standard, Matrix creates yet another standards fork and dilutes everything that came before.
We have to get to the point where progress in messaging is incremental, not revolutionary.
How do you tell that an open standard is "dead"? There are zillions of XMPP servers around with lots of people quietly using them. For a standard to be "alive" does there have to be a large revenue stream associated with it? Does it need a large commercial entity promoting it?
