A person with a knife and intent is just a lethal as someone with a gun. It only takes a second or two for someone to cover 20ft and give you a deadly wound.
Then why do the police need guns? Shouldn't knives be sufficient?
Sure, if your plan is to only deal with threats that are < 20ft away and can be handled with a knife.
I never said the person with the knife would survive. They may very well get killed. Who cares if he managed to slice open your arm or neck and you bleed to death anyway.