What generic message delivery system are you talking about, though? SMTP is optimized for the use case of email; XMPP is optimized for one-to-one instant messa

What generic message delivery system are you talking about, though? SMTP is optimized for the use case of email; XMPP is optimized for one-to-one instant messaging; Matrix is optimized for "Slack-like" chat. I highly doubt that these three use cases are similar enough to warrant introducing an additional common protocol/layer.