That's true of all messaging apps besides base text messages, though, isn't it? It's not like I can send a message on facebook messenger to somebody who reads it in telegram.

RCS / JOIN is an open standard, but it lacks important features like E2E

you don't seem to know Matrix and XMPP

It's very difficult not to be if you want truly secure messaging.

It seems unlikely that big tech and telecom corporations, whose lifeblood is personal data harvesting, or government, with surveillance states becoming the global norm, are going to agree to adopt a genuinely secure end to end encrypted messaging standard or protocol on the best devices ever to facilitate their practices.