This is a dumb questions, please excuse the ignorance: Why were/are there not (or are there?) adapters so that you can speak whatever protocol you prefer (smtp, xmpp x.400 etc) but it still gets through to the user on the other end?
IRC has bitlbee (and probably others) to fulfill that role https://www.bitlbee.org/main.php/news.r.html
Matrix has "bridges" to interop with other protocols https://matrix.org/bridges/
But it's very hard, technically, to maintain interfaces to a changing set of evolving third-party protocols, and provide a good user experience. Especially if those people don't want you bridging to them. Open networks tend to fare better in the long term, but generally the user experience of an app bespoke for the target protocol is nearly always better.