Absolutely! There's nothing wrong with Protonmail's offering, it's just that the other players' dominance will keep that offering from becoming mainstream.
If Gmail started supporting GPG then pretty much everyone would implement it.
Google would not give the keys to the users, so they would still technically be able to decrypt your emails.
I don't think they would implement a security feature that is based on user password (in protonmail you loose your mails if you loose your password).
If Gmail started supporting GPG then pretty much everyone would implement it.