Why not just install a cockpit door that is impenetrable?

Because it is impossible. Aircraft manufacturers have clearly stated that even the strongest possible door will not stop determined attackers. Serious technical issues dealing with a loss of cabin pressure and practical issues concerning incapacitation or seizure of a pilot put limits on how strong the door can be.

A much stronger cockpit door than those that now exist is clearly required, but we cannot rely on any cockpit door to deter or prevent a cockpit takeover from a team of terrorists. A significantly strengthened door will slow attackers down, giving an armed cockpit crew warning and preparation time to defend their airplane. More.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.