The spy in the cab (FD) is seen as a way to 'hang the guilty b*****d'
The spy can also exonerate. In a road traffic accident it proved the obvious suspect was wholly innocent. In the cockpit it can prove all the correct actions were taken. In a tight situation the pilot's hands never left the yoke (joke warning
)
Then think on of the dozens of aircraft where the cockpit door doesn't even exist.
Helicopter operations to oil rigs, sight seeing tours, light aircraft etc all have the ability to be crashed. The only difference is their kinetic effect compared with a large airliner. The door is clearly an irrelevance as regards pilot suicide and passenger safety.
What is different is the kinetic effect rather than the number of passengers. Those therefore calling the locked door as irrelevant and a CC in the FD increasing the danger from the kinetic risk are logically correct.
Pilot screening might mitigate the threat from suicide but would the cure be worse than the bite? More medical s, better compensation etc would drive up costs, reduce passenger numbers which would drive up costs which would . . .
Or would the majority accept the risk?