IM can work the same way, by tacking it on top of email: https://delta.chat/en/
(Bonus: encryption is done automatically for you, by default)
Re:XMPP works – 1 July 2020
by parliament32 on Thursday July 09, 2020 @12:55PM
(#23782213)
Email is not a synchronous channel, and shouldn't be treated as such. Plenty of mailservers will happily delay your messages for a few minutes for spam and virus scanning (because who cares if an email arrives a minute later), and you have all sorts of asynchronous things built into the major servers and protocols (queues, delays/greylisting, 4-day(!!) backoff-retries, etc).
Most of the IM of today isn't synchronous as well. That's the exact reason OTR was deemed not enough, because it requires synchronization, and the Open Whisper guys came up with a novel encryption scheme that works with asynchronicity in mind.
Now of course that doesn't mean you should never be synchronous, and it is true email might not be the best vector here. But from my little experience it's good enough for simple messaging.
Think of the value of this! Anyone, anywhere, using whatever client they choose, can send a basic message to anyone and expect it to arrive and be taken seriously.
Imagine if IM had worked the same way! I know XMPP is a shitstorm, but one can dream.
Not until I left academia and started working for MegaCorp that mandates outlook web as the only email client did I truly internalize the value of this.