You know you're in trouble when people start talking about forward secrecy being problematic. What you're saying about the "email-like use case" for cryptography is that it's unserious protection, because a lack of forward secrecy practically guarantees full decryption of the entire history of messages, for any ordinary participant in the system.
Sure. Because people overwhelmingly aren't relying on the security of their email; it's overwhelmingly stuff no adversary would care to read. Then they retrofit the UX requirements they have for those boring mails onto all emails, and suggest that encrypted email should just accept those as constraints, and then we'll declare victory.
Eventually a private key will leak, and without forward secrecy, that private key will probably decrypt all past messages to that person, and all future messages to that person, until they give all their correspondents a new key.
With email, because people quote when replying, you'll get the other side's messages too.
Like, the simple PGP-like system where sender encrypts message using recipient's public RSA key.
And of course it's not improved by switching from RSA to ECIES.
You need to ratchet the key, or double ratchet like Signal protocol.
