It is a good job Pprune did not exist on 9th Feb 1982 when a Japan airlines DC8 Captain suffering from mental illness deliberately selected reverse thrust on approach on engines number 2 and 3 despite the flight engineer and co-pilot physically trying to stop him. The DC8 crashed into Tokyo bay and 24 people died. The Captain was found not guilty of murder due insanity.
Japan Airlines Flight 350 - Wikipedia, the free encyclopedia
The Pprune readers of the day would doubtless have come up with suggestions of a minimum of 4 people in the flight deck and automatics that would stop the Captain having control of the aircraft. Also a flight attendant on the flight deck for every landing briefed to restrain errant pilots etc.
In practice the rest of the industry carried on normally, reluctant to change worldwide established procedures due to a very rare incident knowing the world would quickly forget. Unfortunately in these days of mass media communication there is much hysteria and calls for kneejerk reactions. No system of door locking is perfect for keeping the bad guys out and the good guys in. All safety is a compromise.