It is perfectly possible to render someone unconscious with a single blow. While I can understand people bring up their professional fighting experience, t needs to be remembered that you're intentionally trying not to get hit. Trying to move so that even when struck the full force of the blow isn't taken. If you watch fights that do end in a single blow, you'll notice how clean the blow is, how the fist lands against their jaw/neck. So its perfectly fine IMO, and frankly it makes people more cautious of having someone get close.