Ok, my take on it, he is in my house, with a knife, he isn't there to play pattycake. He is armed with a deadly weapon, so he is a threat. In my case, my dogs are beagles, they aren't going to stop him. My 12 gage will.
The five foot rule is based on someone with a gun drawn and pointed. They could still kill you even after you shoot them, as was said, unless it is a head shot or spinal cord shot.
Anyone who has had training will tell you, if a guy with a knife gets within 5 feet of you, he has the ADVANTAGE. Unless you get lucky and do a 1 shot drop, you are going to get cut. And maybe dead.
When I lived in Spain in the 70's, the Guardia (Spanish police) would have shot him. Legally. They had the ability to decide to shoot someone they didn't want to mess around with anymore, and often did. You could safely walk the streets in major cities then, no matter the hour. I understand...