I was going to suggest this in my post. A "dead-man"'s limit. A plane would lock you out and right itself it detected you dropped below some pre-defined altitud

I was going to suggest this in my post. A "dead-man"'s limit. A plane would lock you out and right itself it detected you dropped below some pre-defined altitude (e.g. within an acceptable angle without landing gear deployed or a greater than acceptable angle with or without landing gear deployed). Whatever altitude is necessary for the plane to right itself from the steepest descent. Something like that would make it impossible for someone to storm a cockpit and nose-dive a plane. It would need appropriate remote electronic and mechanical overrides, though to prevent possibly losing a plane if it was triggered incorrectly. But at least a plane flying level on lockout autopilot would buy some time opposed to it definitely being destroyed by the nose-dive.