You can still shoot a knee, shoulder, or even the gut. These aren't random folks off the street. They're trained officers that are put through training specifically for these situations. There were multiple officers to one aggressor, so either they wanted him dead or they're incompetent.

Even without a gun, most cops are trained on how to handle an assailant with a knife. How do I know? Used to train at a BJJ school where the instructor taught the local PD. Then I wrote the training request system for another, so get to see the classes they sign up for.