Interesting (and I've certainly played through some pretty terrible injuries myself) but aberrant and not illustrative of standard trauma implications while in the middle of a martial contest (warfare or sport). If a tough guy gets his calf or achilles torn by greater than 50 %, he is effectively lame. If his bicep tendon/muscle or labram tears, he can't wield a weapon/shield/implement with any level of effectiveness. If his blood pressure drops dramatically in a few short moments, he will lose consciousness. Death will follow shortly thereafter where he either bleeds out or is killed (in warfare).
While being stabbed with a sword was generally debilitating and lethal in earlier eras, generally not in the time frame of mere seconds. People stabbed would take minutes, hours or even months to die depending what actually killed them. The main things that D&D doesn't model about injuries that are relevant here are blood loss and infection. If we really want to be realistic, we are going to have to start modelling all those secondary effects of injury that are really what kills you. Sure, if someone put a warsword in someones belly button point up and lifted, the resulting loss of blood as thier insides gushed all over you would kill them pretty fast. But that's not how hit points work. Hit points don't suggest and have never suggested that you are able to endure multiple disembowelments and decapitations.
Sure, cases like that are somewhat rare. But they aren't as rare as you'd indicate. If bleeding stablizes, a person can endure pain and traumatic injury far beyond what people would expect. I watched my brother win the state wrestling championship with his shoulder torn out of its socket. I have a great-grand uncle that owned a bank in Texas, and got in a feud with his neighbors the dry cleaners and challenged them to go out into the street to settle it. He did. They did - but three dry cleaning brothers came out with rifles. They shot him six times and he still killed the three of them with pistols, and then lived for three months before the infection killed him. If you look at stories of warfare you'll find innumerable examples of persons with multiple serious wounds performing heroic actions - numerous persons winning purple hearts and medals for bravery with the same action. In ancient warfare, beserk and fanatical warriors would take what would probably be lethal wounds and keep fighting for minutes.
On a casually realistic level, the relative rarity of such things is easily explained. Most persons are 1st level commoners or the like with just 2-4 hit points. They can't evade or endure weapon blows. Some people have both high constitution to endure multiple blows, and also the experience to evade them. A character that's say a 3rd level fighter with a 15 Con - far above an average person - has both greater ability to endure injury (my brother could keep wrestling with his shoulder dislocated because his deltoid was so strong, he benched over 450lb at the time, it kept his arm immobilzed) and greater ability to evade injury by slipping otherwise telling blows.
You put me in the ring with a professional boxer, it will be one punch and I'll be on the floor. But you put that same boxer up against another professional boxer, and they'll pound each other until they look like hamburger meat. Then at some point in the fight, too weary to remain wary, half-blinded by swelling, and with reflexes dulled by pain, one of them will do exactly what I did in the hypothetical fight - stay still or move into a punch they didn't see coming - and it will be lights out for them too. That's the casual realism of the hit point model.
It's not intended to be perfectly realistic. But the range of outcomes it produces largely overlaps the expecations of casual narration. There are plenty of edge cases where it is clear it doesn't work really well - it doesn't really know what to do with arrows, and commoners vs. housecats or housecats vs mice are problems, for example - but largely the narration we can produce from D&D is plausible.
My D&D is a lot closer to Indiana Jones than it is Band of Brothers....
If there is any character in fiction who could easily be simulated with hit points, it's got to be Indy. I can understand lots of objection to the idea that hit points don't model 16th century reality very well were most death involves slow painful bleeding out from torso injuries, lungs filling with blood, and death by gangreen - but as far as modelling an action movie where the hero can endure traumatic experience after traumatic experience while not slowing down and only ending up with scrapes and bruises, hit points do very very well.
I think you are exagerrating the problems in casual realism produced by hit points. So long as character level is low they produce a pretty decent narrative. If you really wanted to make them more realistic, bleeding rules, shock, and infection could be added to the mixture. In general I find that going that route generates relatively unfun book keeping, death spirals, and so forth while having relatively minor impacts if you've got a cleric hanging around to close wounds and cure disease when the adrenalyn wears off. If character level is high, then the character ceases to be related to some ordinary rael person, and becomes a stand in for heroes of fiction. Rambo can jump from the cliff and fall 100' with a reasonable expectation of only minor injury. John McClane can be punched, shot, and get glass in his feet and still perform olympic atheletic feats. And that's before we even hit 'high level' D&D with its Achilles and Heracles level heroes.
