Hate to burst your bubble there comrade but if someone pulls a knife on you, in most states that is indeed consdiered deadly force and your use of your gun will be considered self-defense, since it is just that.Most people robbing stores aren't looking at gun stats; they look for a small store with few people in it. Moreover, most clerks are told by their employers not to resist. If someone robs you at knifepoint and you draw a gun, you have limited options; unless they lunge at you, you can't shoot them anyway in most states. You are free to use equal force, not superior force, unless you are being actively threatened physically. We recently had a case here in which this happened, and ONLY because the security film showed the robber lunging at the clerk was the clerk not arrested for shooting him.
BTW--there are many people who refuse to wear or use guns.
In your toadstool pagan utopia people may pull knives on you to peel your apple or for kinky pagan gay sex but in the real world, someone threatens with a knife they have forfeited any defense against the use of a firearm.