At 21 feet you prefer the killer have a gun aimed at you rather than holding a razor knife?
I don’t.
Your examples of knife wounds in surprise attacks, or any other attack ,do not make them better than a gun, would those surprised victims be better off having a pistol shoved up against their bodies and the magazine emptied into them?
the 21-foot rule means you are putting your life in danger if the knives are allowed closer, I prefer holding a social worker in front of me.