[Jonty30]

They should have found the body if he hadn’t. He wouldn’t have been that far from where they found the money if he hadn’t survived.

I think he survived and abandoned the money that he dropped.

[Rockingham]

I think that the money was planted as a misdirection to suggest that Cooper was dead. Clued in by the military and CIA, the FBI learned pretty quickly who D. B. Cooper was and, at the highest levels, ran a pretend investigation as part of a coverup.

The reason was that Cooper had been part of highly secret paramilitary missions overseas and could not be prosecuted. The hijacking was Cooper's way of getting pay that he believed he was due.