I opened cockpit door while being attacked, says pilot of Dubai-Israel flight
bbc.com
bbc.com
This is but one of myriad scenarios. "Why not just X" ignores the amount of thought that has to go into security design on aircraft.
They clearly didn't have this case covered.
I assume that there are several pressure sensors inside of the plane, and that its value could be checked real quick by a computer just in case. Lack of pressure plus descending, confirm with both pilot and non cabin personnel, normal pressure plus descending and unresponsive pilot: unlock.
More likely they considered it and determined it to be a remote enough chance relative to a passenger being the hijacker that keeping the door locked was the better default. A LOT of things have to happen correctly (or incorrectly depending on perspective) for a fully trained pilot to decide to and successfully attack the other pilot and take down the aircraft.
Just one. This reminds me the old joke of the $100000 safe password device and the $5 wrench.
The inconvenient truth is that passengers are not the most dangerous people in a plane. Pilots are. They have in their hands the bigger weapon, and know how to drive it to a target. This is not the first time that a pilot goes rogue and locks the cabin, and we are exactly in the same point. Waiting for a stroke of good luck.
What has changed since the last time where they promised to review the protocols?.
If the people at charge can't imagine the possibility of a pilot being given the option of "you die, or you and your entire family dies", or forgot about the suicidal pilot a few years ago, then they really need to quit their jobs.