TSA is not the only line of defense.
Passengers, post 9/11 are anything but defenseless. I think many people would be shocked just what passengers have done to their fellow passengers when they have done stupid things on airplanes after 9/11. It would take a mighty foolish individual to jump up and take a screaming run at a flight deck door today. God help the fool if the plane is on it's way to an airport near a large military base and the plane is full of young and vigorous members of our military. There would be a world of hurt delivered unto the wicked on that flight.