You have to log into Protonmail's web UI with a username and password, too. How do you think this works? I'm not saying you should transmit your PGP key's passphrase to them.

It doesn't work how you seem to think it does. Your password never goes to the server: https://protonmail.com/blog/encrypted_email_authentication/

I see. Still, this process can be performed by a mail client using IMAP or SMTP extensions, the same way the web client does - I would be happy to implement this in my own IMAP/SMTP-related software. Google also has a feature which supports a separate IMAP/SMTP password from your account password, a similar system would be easy to implement for Protonmail.

why go through all that effort (writing a spec, getting third party email clients on board, etc.), when they can write a webapp and call it a day? i'm not sure about other people, but i don't even use a mail client; i just use whatever webapp my mailbox provides.

Many hundreds of thousands if not millions of users utilize IMAP to receive email on their phone apps, thunderbird, outlook, etc. Support for it is expected of any mail provider as part of the basic requirements of operating an email service. Extending this with an extra means of authentication is comparatively easy to adding it to your web app.