While there is a point to be made about PGP not being user-friendly, what's wrong with having a free ProtonMail account (or creating a burner account if you're

While there is a point to be made about PGP not being user-friendly, what's wrong with having a free ProtonMail account (or creating a burner account if you're paranoid), uploading the other person's public key, and having an encrypted email exchange that way? Yes... you're having to trust that ProtonMail is actually secure but I haven't seen anyone seriously suggest that it's compromised. That said, how hard would it be to create a fork of Thunderbird that has all of the PGP stuff build in and all you need to do is upload your private key and add a contact's public key in their address book and have an option for "always use encrypted email" (or does Thunderbird already do exactly that and I don't know because I don't use it)?