Hand-wringing about "what ifs" when a Germanwings F/O took out a whole plane full of people due to this "feature".
If you have someone sitting next to you and they’re determined to crash the aeroplane, then there is little to nothing you can do about it. The answer is to try and stop mentally ill people getting into that position in the first place. There are times when two or more pilots working *together* have a right job keeping the aircraft from doing unpleasant things and on occasions they fail at that.
The whole point of the locked door and the procedures surrounding it, is that those on the flight deck side of it can deny access to those in the cabin, no matter what knowledge they have of said procedures or what they are doing, and that includes holding crew members or passenger to ransom. If you implement master keys, codes or whatever so the door can be forced open then you might as well not bother fitting one in the first place, as you have negated most of its function as well as the deterrence effect. Germanwings showed those who would wish harm upon us that the door was actually secure against someone who knew intimately how it worked.