Again: without forward secrecy, you can't delete old messages, because your adversary has already recorded them. The point is that a lack of forward secrecy creates a subtle limit to the security that can be achieved.
Yes, that's exactly what we both agreed on 6 comments ago. The question is, is the ability to delete messages critical enough to require in any possible secure messaging solution, at the expense of features like email search, archiving, backup, and transfer-to-new-device?