I am a 737 Captain for a major operator and we don't have any expensive security equipment such as cameras to look in the cabin or double doors etc.
When a pilot needs to leave for the toilet we follow a very sensible procedure to identify who is at the door through use of a communication protocol and visual verification (peephole). The Cabin is secured and the pilot then leaves the flight deck (whilst the other pilot remains ALWAYS at the controls) a member of the cabin crew then passes into the flight deck and the door is re secured. When the pilot is ready to come back the same procedure is followed. The Pilot Flying when ready asks the cabin crew member to visually identify through the peep hole and then simply opens the door at which point they swap over again. Both pilots are now back on the flight deck. It is my understanding that this procedure is predominantly to substitute the use of cameras etc as one pilot must ALWAYS remain at the controls during flight. This procedure however also ensures that TWO people are ALWAYS on the flight deck. Should anything go wrong with the pilot flying such as falling unconscious or God forbid some sort of extremely erratic behaviour (like wanting to kill everyone) then the cabin crew member simply turns the door handle from inside the flight deck at any time and this mechanically opens the door outwards to the cabin enabling others to gain access.
I cannot speak for other airlines but my understanding is that if they have cameras etc then there isn't necessarily a requirement for a procedure to keep TWO people on the flight deck. Crews from other airlines will be able to confirm whether this is the case.
The whole point of the expensive cameras and door lock system is to avoid and mitigate the system you highlight. If these systems fail we go back to your system. This would be at the heart of why going back to your system for all is not likely acceptable - it makes clear that we up front cannot be trusted, and that undermines much of our industry's safety foundation.
Not saying of course it will not happen as a kneejerk response.