There's plenty of techniques to take down a gun too, really. Someone just has to have the time/ability to do so. If you don't see a guy coming and he wants you dead, it doesn't really matter if he's got a knife or gun. From there though, situations get bogged down in the details of what is actually occuring and there's just no predicting. Not that a gun isn't more efficient, but that goes both ways. I wouldn't recommend a cop keeps his gun holstered when the criminal "only has a knife".