Speaking for myself as a civilian. It is ALWAYS going to be cheaper and less trouble if you can avoid shooting some scumbag than if you have to shoot them to stop them from killing you.
I understand this cops actions, that while he was fully justified in shooting the guy throughout most of the encounter, he was able to contain the threat and no have to end up in a big inquiry and possible media hanging, etc.
The law in my state says IMMEDIATE threat. That judgement is subjective. The fact the guy is in range with a knife is certainly immediate and justifiable for defensive action. But in the officers judgement it wasn't immediately necessary to shoot the BG.
Could the officer have stopped a charge with a mag dump on the BG if he had charged with the knife? Who knows. Probably, maybe, maybe not. Getting a central nervous shot on a charging, running man is luck not skill for even a top competition shooter. That is why people usually don't take chances.
PS - Don't bring a knife to a gunfight.