This is a counterfeit argument. PGP loses "deniability" (and "forward secrecy") if by PGP you mean "the PGP user interface". But if what you mean is simply "the PGP cryptosystem" and "the PGP message format of packets and bulk encryption and signatures", then you can grant your system most any property OTR gives you.
This is a moot point, because most systems would never care enough to intricately position all their features just-so to compose OTR-like features out of PGP primitives. What they need is to be able to encrypt anything without implementing trivially exploitable crypto vulnerabilities that were discovered and solved decades ago.
This is a textbook case of everyone's good being strangled by someone's opinion of the perfect.
I don't disagree with you. And OTR is just one example, and may even be a straw-man by now. Just be aware that there are other valid choices for cryptosystems, while you still don't have to roll your own.
This is a moot point, because most systems would never care enough to intricately position all their features just-so to compose OTR-like features out of PGP primitives. What they need is to be able to encrypt anything without implementing trivially exploitable crypto vulnerabilities that were discovered and solved decades ago.
This is a textbook case of everyone's good being strangled by someone's opinion of the perfect.