The guy you have to worry about with a knife is not the guy you see coming. It is the knife you do not see coming.
You can't protect from everything. We can also be struck by lightning or a car driven by a texting driver crossing the center line.
In reality, if we are confronted by someone with a knife, or other weapon its in a mugging, car jacking, home invasion, type incident.
In these cases the bandit makes a demand and expects you to follow that demand.
In that case, you are at an advantage, you act, he has to re-act to your action.
Go back top the "clapping hands" drill I mentioned. You can easily get to the point where you can get your (training) gun out fast enough where he claps, ending up with the gun between his hands.
In this case the one clapping the hands KNOWS you are going to draw and still cant beat you. How can someone who isn't expecting you to draw going to beat you?