Ok, let's suppose you can win by punching to the head. It's still ineffective for self defense.
If i ran up on you and started punching you unexpectedly, you're not going to have time to "fight." I'm not going to let you get into your stance and spar. That's how real life attacks work.
Sparring isn't self defence, I never even hinted that it was. It's a game with rules. The problem is that you don't seem to understand it being a game and you seem to think the rules for that game dictate the entire art, even though you don't actually know the rules for every version of the game anyway.
But OK then, I'll play along with a scenario or 4... I'll only use techniques from tkd.
If you run up to me and start punching, one thing that sparring will have helped with is not emotionally falling apart due to being punched. (That's if you don't get a lucky suckerpunch in and get a knock out, for which there is honestly no defence at all)
So, scenario A - I wrap your arm (using the movement trained in an outward forearm block), then with the other hand do an archand strike to your neck while sweeping your leg with a low section hook kick and dump you on your back. Then I follow up with a pressing kick to your face and finally stomp and restomp the groin.
Scenario B. I do a rising wedging block to open your arms and grab your shoulders, headbutt to the face, upward knee to the groin. Then use a circular block motion to lift your leg and drop you. Finally, stomp and restomp the groin.
Scenario C. As you start punching I jump back and do a snap kick to your chin. Then a reverse hook kick to the back of your head, maintaining that foot contact to drive your face into the floor. Now you're down there I can walk around to stomp and restomp the groin.
Scenario D. I sidestep and spin with an elbow to your neck. Then a side thrusting kick to your chest to push you back a bit so I have range to use the wall I was leaning against when you attacked to do a reflex turning kick to your head. Really, that'll put you down. Stomping and restomping the groin would maybe seem superfluous at this point, but I'll do it anyway. Because you deserve it.
Notice how none of them required me to "get into my stance".