Why is non-PFS the default? You wrote that somewhere in the documentation.

You said something about violating expectations when you sync PFS'd chat contents, but I don't see why that's relevant unless people were promised otherwise.

There's nothing about the encryption algorithm itself that dictates how data going over it should be handled at the endpoints.

Just let users have a regular mode and an off-the-record mode where nothing is kept. Both PFS protected.