The reality needs to be part of the discourse, but is not. A knife wielding attacker who is actively/purposefully moving toward or charging the person with a holstered pistol presents a lethal threat at 35-40 feet. (The old and ill-named 21 foot rule, which is not a rule but a description of experiments done over 30 years ago is FAR too close.) Once a person standing still transitions to movement at that distance, the time needed to draw and fire is just barely within the capacity of most adequately trained persons. Remember that the ballistic reality is that pistols suck, and that most often, even with good placement, multiple rounds will be needed.
He only had a knife! - Deputy Matt & Others Who Serve