I don't think a well-integrated PGP-over-SMTP client would leak any more metadata than the likes of Signal does? Build in a good subkey rotation config and you'd solve most of the forward secrecy issues, and good defaults for how to treat revocation (including better expiry defaults) would resolve that issue. No?
I do not know enough to be sure about your point about forward secrecy. You may be right.
Sure, so any intermediate server would see who was talking to who. But that's the case with Signal et al as well isn't it?
No.
”Because your phone will be connecting to Signal’s servers, your cellular carrier can determine whether or not you are using the service. However, your carrier cannot gather any information about the individuals or groups with whom you are communicating.”
Source: https://github.com/WhisperSystems/Signal-iOS/wiki/FAQ#what-a...
But PGP-over-SMTP would still leak important metadata, and you would still have problems with forward secrecy and key revocation.
Matrix looks like a much better decentralized solution to build a new email infrastructure on. But there are still metadata leakage issues with federation, and there need to be some standards and an example implementation for email-over-matrix.