The problem with ProtonMail is that they are convincing people that email can be secure. It could be between ProtonMail addresses, but not with the email protocol and OpenPGP. It would take a lot more work.

Signal on the other hand, it's not perfect and their protocol can't handle larger groups, but they worked hard on it and it shows.

That being said Messaging Layer Security (MLS) is going to be the next big thing for secure messaging, and it's sponsored by companies like Google, Facebook and Twitter, so it's not black and white when it comes to privacy. Apple and Google for example uses differential privacy in iOS and Chrome. We can probably thank GDPR for it, and Facebook for screwing the pooch so spectacularly.