I really don’t know.
Maybe close to 10 years ago I had a friend who hated email so much that nearly any alternative was preferable. To her, DMs on Twitter (must have been early days for that!) were better. Perhaps the short and threaded nature of them was appealing. More likely, the idea that you could control you could DM you by requiring it to be a mutual follow.
That’s not really a thing with email. I suppose you could write email filters that block all emails from people not on some particular list, but I’ve literally never even heard of anyone doing that. Email is this open door, with all the good and bad that comes with that. That’s not to say a more closed-door communication method isn’t valuable, it is, but it doesn’t seem to be unseating email.
Google Wave notoriously was meant to be “what email would be if email was invented today”, which was nearly 10 years ago now, and is dead as a doornail, and nobody has even tried to do anything as ambitious since.
I remember the early days of Slack had marketing that eluded to Slack being something of an email replacement for business communication. Perhaps to some degree, but as much as Slack is talked about as being this vital tool for business today, people don’t seem to be any less burdened by email.
I don’t have any big ideas, I just think it’s interesting. Tech is this place where there are always new companies trying to disrupt huge markets, and email is about as big as it gets. It seems like the closest stab at it is fancy email clients, but even the fanciest features largely just light UI and UX improvements over the core features of email.
The fact that email is “free” is probably somewhat of a turnoff for starry eyed innovators. But I’d posit that email has done such a good job at what it does, it doesn’t need to be unseated.