Wire was able to implement a fully E2E-encrypted messenger with proper multi-device support almost a decade ago, long before it became mainstream. Fully FOSS too, including the server. For some reason it never became popular. They don't have proper desktop clients (just the usual Electron mess), but then, which one of them does except for Telegram?
My memory is really fuzzy, but I recall back when the new IM apps were all coming out and competing for the small share of people wanting to test them, either the Wire app or using the Wire service had a hard cost ($5 USD?) (no free-to-test tier, you had to pay to use). I believe history has shown a vast majority of folks will choose free (even with ads, sadly) over a hard cost for IM. Signal talked a better game and offered everything for free.
I moved all my friends to Wire about a decade ago. Pretty, but the actual user experience was awful. We moved to Telegram and since then to Signal.
I didn't actually know Wire was FOSS.
How do they handle message recovery when all your devices have been lost (and possibly when you've forgotten your password)?
That's (almost) incompatible with E2E encryption. You could do it via "social proof" or server-side secure enclaves and pins, but that's about it.
I mean, E2E is the entire point of Signal. if you don't think E2E is important then Signal is the wrong app. Personally given the current political climate I think having the technical knowledge to understand what E2E is and not wanting E2E is bonkers. High chance of people getting killed or jailed in the US for mainstream political positions in the near future.
Most apps on the market are E2E by default these days, and that introduces a whole host of complications. It's the wrong tradeoff for 95+ percent of users. If you can only afford 1 device and only switch to a new one when the old device breaks, E2E is a disaster in the making. For the overwhelming majority of users, making sure that they have access to their messages when they switch devices is far more important than being protected from the NSA. This is something most signal advocates are completely unwilling to talk about.