You’re underestimating the costs of E2EE even in messaging. There are plenty more that cause trouble (e.g. lack of recoverability, no content-based spam filteri

You’re underestimating the costs of E2EE even in messaging. There are plenty more that cause trouble (e.g. lack of recoverability, no content-based spam filtering, various key sharing problems), but I’ll focus on one that’s probably the easiest cost to talk about: no server-side search. If you want useful search, you have to download the entire collection, and be bound by your client’s performance. (In theory you can just download/store/upload an index, but in practice I gather that isn’t workable and doesn’t help anything.) This is generally tolerable for something like iMessages which is designed to be used on one or a very few physical devices where it’s persistently stored, but is very impractical for things like web access and larger collections, things like email. By most people’s threat models, E2EE for email is a terrible idea.