All this speculation on door security or automation is moot. IF you cannot trust a pilot to always act with the safety of his/her aircraft as first priority and those on the ground second... then what?
My career was ATC. You pilots know your side, but a determined individual sitting up front can kill with certainty on takeoff or short final, ( esp in wx or IMC ) a lot more than cruise flight regardless of another pilot in cockpit or not. And too.. an Airbus would not react well to left seat fighting the right seat for control, no?
No one can automate out intent. Once we go down the rabbit hole of not trusting one pilot actions... commercial aviation is doomed. Unacceptable risk.