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.