yabai is quite wrong about the vulnerability of
strong encryption as anyone who's versed in the mathematics involved knows. However, there is another, much bigger vulnerability in the concept: what about
interference? Instead of taking over the plane, one could just bombard it with
white noise on the control frequency. During complex maneuvers such as starting and landing, this would hav almost certainly catastrophic results. No, there are good reasons not to trust technology too far.
I actually read the other suggestion (remove communications between cockpit and passenger cabin) before, in a book from the 70ies written by Paul Watzlawick. He said it was infeasible because there were situations were such communication was absolutely necessary. The "emergency button" sounds like an interesting solution, but I'm not sure if it would be really sufficient.