Commitment Strategies vs Highjacking
On the face of it, almost all of the precautions to keep passsengers from highjacking an airplane are unnecessary—all it takes is a reasonably sturdy locked door between pilots and passengers. One possible response is that highjackers might persuade pilots to open such a door by threatening to kill off crew and passengers one by one until they do. In the post 9/11 world I'm not sure that would work—but suppose it would.
There is a simple solution: Provide the pilots with a second lock that can only be unlocked by someone on the ground. At the first sign of a highjacking they lock it and are now immune to threats. It would be prudent to make sure that potential highjackers know about the second lock, and perhaps to have a lighted sign or prerecorded announcement to signal that it has been activated.