Regarding the idea of FA's suddenly becoming threats in cases of flight deck crew breaks is a little strange. These are the same crew that supply hot coffee and (sometimes) food and are routinely granted access to the flight deck. If we trust them for that job, I am not sure how sitting in the jump seat for a few minutes is going to provide much more of an opportunity for one hell bent on ill intent.
Some might argue that routine access to the flight deck would mostly occur when there are two pilots present. If you have a regulation that requires a non-pilot to be there at all times when there is only one pilot at the controls, you have created a situation with a higher vulnerability to a “sleeper”. There is also increased opening of the FD door in a predictable manner. If you were paranoid, you might want it the opposite way round, i.e. access to the FD is only allowed when two pilots are there.
The airlines that have the “rule of two” are either forced to do so by their regulator or are too cheap to install video surveillance and/or remote door opening so need a third party in there to check through the spy hole and open/shut the door, which a single pilot cannot do without getting up and leaving no-one flying the aeroplane for a while.
To be fair, both the rogue FA and suicidal pilot scenarios are in the extreme low probability end of the spectrum. CFIT, runway excursions, equipment failure, etc. are hugely more popular when it comes to breaking aircraft. This gets overlooked when you have a spectacular accident like the one we are discussing and you get lots of knee-jerk reactions which make the overall job of safely transporting people from A to B more difficult.
If you try and be objective, what increase in safety will you get from having a non-pilot on the FD? What is their function? If it’s to open the door against the wishes of the remaining pilot, the result of that depends entirely on which of the two has malicious intent. Even if it is the pilot who has "gone postal”, how is a non-pilot going tell the difference between that and normal/non-normal operation by SOP? And what are they going to do about it in the limited time available?
The ultimate truth is that a pilot who is determined to crash an aircraft can do so irrespective of how many people are in the cockpit and who they are. You
have to trust pilots to fly the aircraft in a safe manner as there is no alternative. Where attention needs to be focussed is in making sure that those sitting at the controls are in the best possible mental state for doing their job correctly. Unfortunately, the recent trend has been for the amount of stresses, distractions and obstacles put in the way of sensible aviation to be on the increase...