I’m not sure if this is required. Any decent e-mail server uses TLS to communicate these days, so everything in transit is already encrypted.

In transit, yes, but not end-to-end.

One feature that Proton advertises: when you send an email from one Proton mail account to another Proton address, the message is automatically encrypted such that (assuming you trust their client-side code for webmail/bridge) Proton's servers never have access to the message contents for even a moment. When incoming mail hits Proton's SMTP server, Proton technically could (but claims not to) log the unencrypted message contents before encrypting it with the recipient's public key and storing it. That undermines Proton's promise of Proton not having access to your emails. If both parties involved in an email conversation agree to use PGP encryption then they could avoid that risk, and no mail server on either end would have access to anything more than metadata and the initial exchange of public keys, but most humans won't bother doing that key exchange and almost no automated mailers would.

Some standard way of automatically asking a mail server "Does user@proton.me have a PGP public key?" would help on this front as long as the server doesn't reject senders who ignore this feature and send SMTP/TLS as normal without PGP. This still requires trusting that the server doesn't give an incorrect public key but any suspicious behavior on this front would be very noticeable in a way that server-side logging would not be. Users who deem that unacceptable can still use a separate set of PGP keys.