If an individual is threatening you with a knife and you have a firearm, the knife-holder is still extremely lethal, that is why the police are judged to have committed an authorized act when shooting hostile people approaching them with knives. As long as the person doesn't advance on them, they will just keep yelling at him/her/it until either the person puts it down, or they "taser" the person.
You are apparently unaware of the "21ft. rule," when it comes to a dangerous person with a knife. Time and again, it has been shown that someone armed with a knife, can run at and be on you from a distance of 21 feet in 1.5 seconds. That means you have 1.5 seconds to react. Do some basic research and if you still think I'm full of it, go to a toy store and purchase two items:
1. A rubber knife.
2. A toy dart gun (the type with the suction tipped rubber darts).
Measure out 21 feet and have someone (healthy, not wheelchair bound or very elderly), charge at you with the rubber knife. You will be surprised how fast a person can close that distance.
So....a person armed with a knife is indeed a lethal threat and you have to be prepared to act instantly, should the person run at you. It would be nice if you had a taser to use first, but should that be ineffective, as some are, depending upon how "hyped up" they are, a firearm is a good back up.