“The vulnerability to insider threats is huge. How do protection measures get implemented without crippling the industry?”
One way, for the newer plans at least, is to have a list of safe airports to land at, and then give ground control the ability to have the aircraft fly itself to one of those airports, while fully disabling the cockpit.
In other words, if you have a Muslim in the cockpit, without weapons, there’s nothing he would be able to do to meet his 43 virgins or whatever.
In one of Isaac Asimov’s science fiction novels, the people in the future wonder how anyone trusted aircraft in the hands of humans, rather than the much safer computerized, robotic control...
What if the ground controllers are terrorists too?