This is never going to happen because it doesn’t work very well for consumers, and works even less well for businesses.
For consumers, owning the encryption keys means account recovery is impossible when they inevitably lose their keys. IM services can get away with it, because losing your IM history is not nearly as serious as losing your inbox.
For businesses, you’re not going to be able to sell a service that makes filtering impossible. This is bad for consumers too, but an absolute deal breaker for most businesses.
... including spam filtering, which matters somewhat for consumers, too.
Then there's the issue of search - with webmail you have no realistic choice but to rely on server-side search, and the same issue likely applies on phones even when using a dedicated mail app. (And indeed ProtonMail currently only offers meta-data search, but no full text body search)
I didn’t specifically say spam filtering, because there’s technically lots of spam filtering you can do with only metadata. But yeah you’re right, any form of server side content filtering (including search) would be impossible.
For consumers, owning the encryption keys means account recovery is impossible when they inevitably lose their keys. IM services can get away with it, because losing your IM history is not nearly as serious as losing your inbox.
For businesses, you’re not going to be able to sell a service that makes filtering impossible. This is bad for consumers too, but an absolute deal breaker for most businesses.