What if a terrorist beams in from an orbiting spaceship? There's ALWAYS a way to get around defenses.

The object of the game is not to make crime impossible, it's to make it harder for the criminal. The car alarm, the Club on the steering wheel, and the gadget that makes lights go on and off in your house are not to make it impossible for a criminal to break in. The idea is to make the criminal decide it's not worth the risk.

A locked cabin door helps. Knowing that there are guns, or at least MAYBE guns on the other side would REALLY help. The deterrent effect is an important one. In a state where concealed weapons are legal, those who carry weapons rarely advertise it, but criminals know that SOME people do. This deterrent helps even those citizens who don't carry concealed weapons. The ideal solution would be not be for the government to either forbid, or mandate pilot handguns, but to leave it to the pilot's discretion, and insist on certain rules pertaining to gun type, ammo, and other safety considerations.

The hijackers succeeded before because they were doing something different than what was expected. The pilots and passengers behaved in a certain way because they were expecting to land on a runway in Cuba or something, and the hijackers counted on this.

Actually, this is all probably moot. The odds of another airplane-into-a-building attack are slim to none, unless its a private plane with explosives or something instead of a hijacked one, and most very large buildings would be reasonably immune to a small plane. The advantage of surprise has been lost.

The next attack will come from another unexpected direction. The trick is, of course, to try and expect it.