By looking at the history, when there were no restrictions on this we did not have people sucessdfully hijacking aircraft. That is the simple answer. Clearly, if some of the passengers had been armed with firearms and willing to resist there would not have been the aircraft flying into the WTC and the Pentagon.
Perry was on a flight to Europe and the guy in the seat ahead of Mason, accidentally drops his revolver and it slides under the seat where Perry picks it up and simply hands it back to it's owner.
Could anyone imagine a TV show now where that happens?
Let's consider an equivalent of the times; sailing ships. Although no one would question the rights of the passengers & members of the crew to own & carry, once aboard they were under the command of the ship's Captain. Firearms were maintained in an armory or locker until Captain's orders (or mutiny conditions) distributed those weapons. At most, senior officers may have carried a pistol to keep sailors, a sometimes unruly lot, to help keep order. With that in mind, ships were frequently armed with crew served weapons, (cannon/weapons of mass destruction) to guard against attack by other ships, supporting a right and practice of keeping a class of weapons now banned as destructive devices.