> end-to-end encryption has never been shown to work in a real system with normal people

I would argue that Signal is a great example of this working quite well, and tons of normal people use Signal. It's no more frictional than WhatsApp.

Signal is a promising example, but ephemeral messaging is a very different use case from things like photo management.

Signal has a lot of problems with changing devices while preserving history. As in it's often just impossible.