We need to be about the business of making sure that you have the capability of throwing over command of your aircraft to a ground unit that will guide you onto a militarized airfield, where you'll be met by a well equipped and highly trained anti-terrorist team, and providing you with security onboard.
Why do that when it's easier and cheaper to just shoot us down.
Everyone on board will be dead long before the atc guy is able to land the aircraft anyway.
If the bad guys get in the cockpit, it is all over no matter what cockamamy stuff is installed.