Quote from ManaAdaSystem:
"Have we really come that far that a modern jet aircraft becomes unsafe just because you switch off a flight director?"
In the case we are discussing, it is not the switching off of one FD that is responsible for the problem. It is the PF's failure to comply with the pitch commands of the remaining FD that causes the speed to decay. To put it bluntly, if you tell a non-human system to guide you, and then deliberately ignore its advice, there may be consequences. In the original design, the only safety-net to a pilot's mismanagement and inattention was alpha-floor. After Bangalore, AI introduced a second, higher safety-net.