This is a bad security argument. With OTR / FS in general, you can re-encrypt the plaintext using a different local-only key after decrypting the forward-secure

This is a bad security argument. With OTR / FS in general, you can re-encrypt the plaintext using a different local-only key after decrypting the forward-secure ciphertext. Then, the ciphertext that was sent over the wire is still forward-secure, and your adversary must seize your computer in order to try to decrypt the re-encrypted non-FS ciphertext you use for logs/archiving. There really is no reason to use non-FS encryption over FS encryption for ciphertext sent over the wire.