> ProtonMail only works if you send messages to another ProtonMail user.

Your wording is not entirely accurate.

ProtonMail works perfectly well with external PGP. You can import third-party keys and IIRC it also supports WKD.

So its not "only another ProtonMail user".

IN ADDITION .... you can also send an encrypted, password protected message to a non-Proton Mail email address. The recipient can read it in the browser: https://proton.me/support/password-protected-emails

Your use of ProtonMail+PGP is not how people expect ProtonMail to work when it is claimed to be encrypted email. ProtonMail+PGP is no different than GMail+PGP or AOL+PGP or FastMail+PGP or $coolNameMail+PGP. So what's your point?

Anything+PGP can be encrypted. You still have to use PGP which means creating public/private key pairs, store them correctly, and understand when to use the public key and when to use the private key. You still then need to know how to exchange keys in a trustworthy manner, but that's after you convince someone else to do all of that on their end as well.

So yeah, it's dead simple <facepalm>