Except that the popular messaging apps don't have published standards and you can't interoperate with them even if you wanted to. How do you implement the iMessage protocol on Android or Windows?

Point me to the existing IETF RFC for e.g. mapping email addresses as identifiers for use in a standard communications protocol for voice and video calls.

You used to be able to, with tools like Trillian and Gaim.

Not any more, at least for any semi-popular chat. The throwaway and no-names? Yeah you can, well until IF they get big. And well, they won't.