Most national agencies have implemented the “minimum two crew members in the cockpit rule" as a response to EASA SIB No.: 2015-04.
From that day on, every time, one of the pilots needs to leave the cockpit for physiological reasons, a cabin crew member, has to be called to "watch" the other pilot's movements, in order not to allow him/her to lock the door and plunge the aircraft into the sea or into a mountain hill...
Not only this recommendation was immediately implemented by most agencies, without proper investigation and consideration for unforeseen risks, just to appease the public opinion, but was turned into a rule in many countries.
There is nothing an element seated in the first observers seat can do to prevent a suicidal pilot to crash an aircraft! (Every pilot knows that). From take-off to landing phases, there are a miriade of ways to intentionally crash an airliner and so, the ultimate goal should be, to reassure that we have psychologically fit pilots at the controls at any time. Not to have watch dogs at their back.
On the other hand, the permission granted to third persons to enter the cockpit — while there is only one pilot at the controls — even if that person is authorized in accordance with CAT.GEN.MPA.135, leaves that pilot alone, facing forward, completely vulnerable (offering his/her's back), to another person, he/she knows nothing about, and that could well be a potencial suicidal person…
It is a question of statistics, therefore, a question of time: the more people are granted access to the flight deck, the greater is the risk of one of them being a suicidal potential person.
So, the million dollar question is: why grant more people access (while the other pilot is alone in the cockpit), if he/she is unable to prevent a malicious pilot to crash an airliner?
There are no final solutions for the prevention of suicidal pilot's misbehavior. But a good start would be to facilitate a closer relationship between management (Fleet Chief Pilots, GeneralOps Management, HR, psychologists, TRI'/TRE's, SMS, etc) and their working force mates.
A more human and closer relationship done through the promotion of workshops and leisure events and specially through the integration within the workforces of companies while dignifying careers and wages.
A system where a pilot would feel at ease to talk freely about his/her personal life, without being afraid of being judged, penalized (personally and economically), nor prejudiced by being honest.
In my humble opinion, this recommendation is unsafe and doesn’t motivate operators to accept the responsibility to do their homework on the establishment of preventive mesures for a healthy work environment.