I don't get it. Protonmail still doesn't support PGP, yet they're working on open source libraries for other people to implement PGP? I don't understand these priorities.
For internal use we had all RSA keys so it was fine. For external use we have to support what everyone uses, and there is a variety as you know. Saying "hey, you can use PGP but all your friends have to have RSA keys" is not a recipe for a good user experience.
This is great to hear! If/when protonmail transitions to be mail host that support open standards in a wider sense, I'd more seriously consider becoming a customer :-)
Fwiw, while I'm not that interested in the current project - I actually think it's great to build a walled garden on open standards - especially for secure services. Makes it easier to evaluate, and opens the door fore secure interop in the future.
