There is much concentration here on the role of ATC and confliction with TCAS etc.
What I have always found difficult to understand is why both or either crew continued with a course of action which plainly wasn't working. If you are established in a descent in reaction to a TCAS RA or an ATC command, and the relative level of the aircraft is not changing, why do you continue with it?
In broad figures, if a conflicting target indicates +00ft on your TCAS, you start to descend and it remains at or close to +00, then obviously another plan is needed, even of the instruments dictate otherwise. Even a 10 degree turn might have meant they missed each other.
In addition, the vis up there was greater than 10 km. Both aircraft had anti-collision lights and were aware of the other's presence. There was little or no other traffic about, and there was fairly good cloud cover preventing a myriad of ground lights from hiding the lights of the other aircraft. Even allowing for zero relative motion, why were the crews (and there were 4 of them on the 154) not able to look out of the window and see each other in those conditions? Was there (and is there still) perhaps too much reliance these days on the 'gizmos' to solve the problem and less on the basics of pilotage?
No disrespect intended to the deceased crews, but a basic question.