If you are 20 feet away from a knife wielding psycho, and your gun is holstered, you're gonna come out of this second best. The knife wielder will close & cut you before you can draw & fire every time.
Only if he is lucky, everytime. I have seen a lot of not fatal defense wounds. A gun or a knife is only part of the equation. I am not going to stand there like a 1950's western hero waiting.