So people in situations with a specific need for extra security can use them. When publishing it, this meant anyone who 1) explicitly joined the PGP ecosystem and has some understanding of it and 2) explicitly enabled encryption for this specific email or recipient. Neither are the case with what Proton is doing.
Seriously Proton, just add a god damn prompt! And while you're at it, submit an RFC for a "please use by default" field in certs. Bonus points for a "this key is on a yubikey in a safe so if you use it you better have RCE in production to report" field as well or, hell, just a "key description" text field that clients can show the sender before using the key.
Our goal is to make email encryption more usable and less niche, and not only for "situations with a specific need for extra security" - because people shouldn't have to think about which of their emails are and aren't security- or privacy-sensitive.
Could you imagine if individual users had to opt-in to using TLS? Nobody would use it. The large push for using TLS everywhere has helped internet security a lot. Enabling the use of OpenPGP everywhere would also help much more than serving the use case of "this key is on a yubikey in a safe", because almost nobody is going to do that anyway.
TLS is opt-in for each website. If a web server doesn't specifically listen with a TLS certificate, users don't connect with one.
Browsers will often attempt that connection, and then tell the user ~"Hey we tried to connect with HTTPS, but the server didn't respond. Do you want to try an unencrypted connection"
But there's no case where Chrome will look and see "oh look, akerl has published a cert on this other site, we're going to just send traffic encrypted to that cert when we connect to his website".
KOO is opt-in for each recipient. The recipient has to upload a key to KOO.
Obviously, OpenPGP works differently from TLS, and TLS does not have a concept of key servers - but key servers are absolutely not a new invention for OpenPGP, and it shouldn't be surprising that if you upload your key to one, you might get an encrypted email.
Seriously Proton, just add a god damn prompt! And while you're at it, submit an RFC for a "please use by default" field in certs. Bonus points for a "this key is on a yubikey in a safe so if you use it you better have RCE in production to report" field as well or, hell, just a "key description" text field that clients can show the sender before using the key.