Why would a hijacker abandon weapons? If their goal was hijacking and they found they were to be searched then why even board the plane? Just go back home with your weapon.
Another thing to consider is non-hijackers carrying guns for their own reasons. It was fairly common for business travelers to carry a lot of cash, and carry a gun while doing so.
Indeed, that was one of the major shocks (and changes brought by) 9/11: before that in the case of highjacking you wanted to remain calm because you were at relatively little risk of harm, in the early 70s highjacking were a weekly occurrence and treated as inconvenience. Jacking gradually went down through the 80s as security screenings were added.
The danger was in bombings and there was not much you could do as a passenger.