Honestly I'd like to see one of the webmail providers do a decent attempt at gpg. The web migrated from a primarily unencrypted state to an encrypted one - it's not impossible with the right UX.
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)
Yes, it would be possible to encrypt email too but it would involve changing every email client and server there is, and there are quite a few of them. And a public key repository for everyone to be able to find the correct key for each receiving adress. Mailing list servers and other group mail would be particularly fun to solve.
Communication between ProtonMail customers, you still have to trust that their UI hasn't been MiTM'd.