I /imagine/ that one flight crew member having some kind of brain fart has been a problem in commercial cockpits for as long as there have _been_ commercial cockpits. The post war years into the 1960s when perhaps the majority of commercial aircrew in Europe were potential PTSD victims vulnerable to flashbacks from their last combat mission would seem to be a time when if this was ever going to be a problem, it would have been particularly acute... and yet we don't read about Constellations and DC6's being deliberately flown into mountains very often back in the day so what is different now?
Could it be the breach in the rule of three bought in by two person cockpits? As with flight management systems, two is plenty when both are working properly but you need three so that when one breaks down the other two find it easier to spot that there is a problem and can override the malfunctioning unit. Never mind the practical problem of only having one person on the flight deck when the other pilot goes to the toilet, if the guy was not firing on all cylinders prior to the point where he was left alone to do his worst, two people in the cockpit not in the midst of a mental breakdown might have been better placed to spot it than one on his own and instigate precautions that 99999 times out of 100000 would be completely unnecessary paranoia but once in a blue moon would prevent a tragedy...