I'm pretty you're thinking of deniability (from OTR) rather than forward secrecy.
The forward secrecy in protocols like Signal allows for things like disappearing messages, which are then actually technically credible (the sender's and recipient's devices literally don't contain any information which would help to reconstruct the contents of their old messages). I think the former isn't really helpful in your scenario, but the latter is.
You're absolutely right. Still, I'd ask the same question about forward secrecy: is there a realistic model where they actually help? My impression is that people would generally keep their chat logs for as long as they're relevant, and so a message can only ever become unrecoverable by an attacker once it's irrelevant, in which case it's likely to also be irrelevant to an attacker. I guess maybe there are cases where someone is picked up and it's found they were also involved in some otherwise successful action years before?
The forward secrecy in protocols like Signal allows for things like disappearing messages, which are then actually technically credible (the sender's and recipient's devices literally don't contain any information which would help to reconstruct the contents of their old messages). I think the former isn't really helpful in your scenario, but the latter is.