"Now the end-to-end encryption will leak into the UX even more and you better like it"

I'll say it again: E2EE will never become mainstream unless someone somehow manages to implement it such that it's completely transparent to the user while keeping all the features that people have come to expect from IM apps, like server-stored conversation history or support for multiple devices. By "completely transparent" I mean that the user doesn't have to do any extra actions whatsoever to make it work.

If that's true, then E2EE will never become mainstream. Consider this scenario: "My phone got lost/stolen/broken, so I just got a new one. I haven't logged in to this app since I got my last phone, so I forget my credentials for it. I'll reset them through my email. What do you mean my conversation history is gone?"

That's not really far-fetched. If you can get your conversation history back in that scenario, then so can the server operator so it's not real E2EE, and if you can't, then by your statement it won't become mainstream.

I mean we’re there for Signal. The parts that suck still are regarding access/retention of old messages which is an area Matrix is ironically slightly better about. But Signal we don’t need to think about verification, at worst it says this asshole has a new identity and then I have to tell them I’ve reset my iPhone for the 4th time this week…

Normal users do find retention important even if privacy/security minded users find value in ephemerality.