The trivial attack is based on them sending you compromised code, right? Because that seems to comparable to an attack on GPG where their downloads are compromised.

I get that there's a difference in degree, i. e. GPG binaries being checked against hashes and having a long track record as an organization, but is that fundamentally different?

I could see ProtonMail evolving to, for example, using a browser extension that allows you to use a known-good version of the crypto library, and informing you of changes.

Point being: it isn't perfect and I'd prefer something based on standards. But e-mail encryption has failed, even though it is often more personal than websites where TLS has been successful. ProtonMail is a legitimate attempt in a space that seems to need a new approach.