But then by those constraints you can also count HTTP or SSH or Telnet or any other protocol at that layer.
For any end user, email is the universal communication mechanism.
I guess in theory you could leave an HTTP packet for someone to pick up months later, but existing networks would time it out.
(To be honest, the only thing I really like about slack over IRC is history. I wonder if that couldn't be processed in a more peer-to-peer manner over IRC?)
Is it really so bad? Is there anything on the horizon that could replace email as an open messaging protocol?
I guess kilobyte-sized text snippets are probably a pretty good fit for a blockchain-based trustless messaging service. A million monkeys are hacking on that right now, right? Maybe that will just leapfrog everything else?
People got familiar with email because their ISP/employer provided them with an email address. Eventually it became a thing on it's own. Chat was never like that. There was always one service you used, and all your friends used. Now that's gchat and facebook chat.
The original Google Talk ifnrastructure is still up and running and clients can connect to it using a protocol derived from XMPP (basically a subset XMPP converted to protobuf) and use it to message devices (Android uses it push messaging, but technically you can also send messages directly between devices). It's not directly used for text messaging though.
XMPP and Matrix can.