Good thing that is not, and is explicitly not, what hit points have ever represented, in any incarnation of the game.
I agree, which is why I've never tried to map them to an acurate depiction of physical injury.
So, yes, I'll agree that your hypothetical system is as bad as 4e's actual system, where my apparent "mistake" is either trying to describe hit point loss in-world or trying to describe hit point recovery in-world.
The mistake is coming up with a situation that doesn't work, and then blaming the system for that mistake.
You're saying that it's unbelievable that the wound stiches itself up. Yep it is! But where in the game does it indicate this is what happens? YOU are creating that aspect.
Thats the same as if I were to say: "The axe bites deep, causing you to drop to the ground dead" while your characetr has 50 hp left, and then saying dude this game is BUNK! How can you survive that axe hit I described as killing you???
You're describing an action in an unrealistic way and unhappy with the results of your own description.
Yes, sometimes the narrative of the game is directed in part by the rules of the game. If I KNOW you have 50 hp left, I also know I cannot decribe a 10hp axe hit as killing you without ignoring the rules of the game.
I cannot say I jump over the yawning chasm, fail my jump check, and then complain shoedinger's cat shanked me.
In a similar fashion I cannot describe an effect of a hit without taking into account the rest of the rules of the game. If you do so, you're creating the problem, not finding one.
You must either:
1. Accept HP do not represent physical injury, and recovering HP does not represent healing physical wounds.
or
2. Accept HP represents physical wounds but avoid describing things as gaping wounds until the final death blow.
or
3. Be ok with wounds stiching themselves back up without the benefit of magic somehow.
All three are valid, depending on personal taste.
(Of course, when I said that 4e hit points didn't map to in-world events, I was told I was wrong there too. Schrödinger's Arguement about Schrödinger's Wounding? Man, that cat gets around!)
RC
This is because of the power of narrative. Really HP can map to whatever you want them to, you just have to be prepaired for the consequences of your own imagination, and how it interacts with the rules of the game. Some depictions will result in more situations that seem absurd.
Personally I long ago gave up mapping HP to real injury. It just never made sense. I've tried using injury tracking stuff, but it just gets cumbersome in my opinion. You might feel otherwise, and I don't fault you at all. For some it's fun, but in my games I've found it only leads to a lot of laying around waiting for broken bones to heal.