I've tried to adopt Matrix multiple times over years.

IMO the UX is evidence that e2ee is not solved.

we solved it in element x, but there appears to be massive inertia towards adopting it because it doesn’t have threads or spaces yet.

If I only use 1 device for 5 years, then lose it, how do I recover?

From a backup?

Backed up from what? People only use the cloud for backups, at which point the e2ee is compromised.

It doesn't matter what people are currently doing. In Matrix, there is a possibility to make reliable, encrypted backups. It can also be done with a simple interface.