But PFS is specifically about including "my adversary may, at a later date, compromise my private key" in your threat model without giving up plaintext. If we assume that calculating a private key from the public key is ~impossible (which I hope you agree we can do), and we further assume the private key never leaves the device, then forward secrecy is what lets us know the only way to get plaintext is by stealing it from an endpoint. Maybe I'm failing to see the adversary you're defeating with PFS if they're never going to access your device and siphon off private data...
I'm no expert, but if all you care about is transit security, I don't think you need PFS (in E2EE messaging! TLS is a different story, because you have to trust the server). Just protect your private key. But if you're carrying multiple device's worth of accumulated messages _right alongside_ your private key, then what's the point of rotating ephemeral keys after each message?
EDIT: I agree about a compromise on PFS/chat history being reasonable in most scenarios. But I also think that defaults are really important, especially as the contents of chat history can be leaked by other participants, whose chat backups you can't really control. It's a tough problem to solve for everyone.