One thing I’ve always used to depict “Hit Points” as is not necessarily damage, but rather, a character’s ability to avoid serious damage. For example, Tanks have a better sense of how to mitigate whatever damage their armor isn’t fully protecting them from. In this sense, 0 HP is more like actually receiving a real wound; a bullet finally hits, the sword goes through your lung, etc, and that’s why your character really goes down; they take a genuine, serious injury. HP is more like stamina.
To be fair, hit points are a somewhat necessary abstraction to begin with. The ability of the human body to survive horrific injuries doesn’t neatly render down into a single statistic. (Nor a discrete collection of numbers.) At the same time, characters aren’t (usually) invulnerable, and you need a system that can quickly approximate combat.
Since, I don’t think I’ve really discussed this in detail recently: at an abstract level, when you’re writing, you’re playing a game. It’s not incredibly dissimilar from a GM running a tabletop RPG session. You set rules to establish a rough illusion of fairness, and sometimes cheat a bit, to push the story in the direction you want.
When you’re actually playing a game, the rules are concrete and there to provide an element of fairness (or a uniformity of unfairness, depending on the game in question.) Within that context, HP serves a vital function, informing the players exactly how badly they were just mauled, without automatically removing a player from the current play session.
That analogy a minute ago, about writing being a lot like being a GM? I stand by it, but this is one very specific point where you might want to seriously consider what rules you’re working under.
Large hitpoint pools (in relation to the damage received) work better when you’re trying to provide a system that draws things out, and slows down combat. For the most part, this is what D&D, and most D20 based games, do. At higher levels, you’re very unlikely to be one-shot from full health (though it can happen.) Which leads to the exact issue you’re describing a workaround for.
You take a character, with a large pool, empty a shotgun in their face, and they keep ticking. Even when the rules are relayed transparently, that’s going to leave a few people scratching their heads. Thing is, you don’t actually need this (for games or writing.)
What you described is one way to reconcile this without altering the rules. As I recall, it was the official interpretation for D&D at one point. Your HP wasn’t your health, but a measure of your character’s ability to avoid life threatening injuries, and trudge on.
If you’re writing a story about fantasy heroes or superheroes, then this approach makes sense. It fits within the genre conventions. To be fair, when we’re talking about D&D, and a lot of heroic fantasy RPGs, that approach is consistent. Improbable health pools strain credibility, but the ability to just keep fighting is part of the genre.
Now, if you want to chalk some of that up to their armor soaking some of the damage, that’s fine. It is consistent with how D&D, and a lot of games, present combat roles, so that abstraction isn’t really that strange.
Thing is, this only really works if you’re aiming for superhuman characters. If you’re wanting something more grounded, you want a much smaller pool of health (again, in relation to the incoming damage.)
At the extreme end of this, you can get stuff like White Wolf’s storyteller system, where you have seven HP. That’s it. Each point lost indicates specific thresholds of increasingly severe wounds. The game boosts survivability by giving you more opportunities to resist damage, but it creates a situation where any combat encounter has the potential to go horrifically wrong without warning.
When you’re talking about armor, you’re not going to be fully protected. Even if you have a character in full plate, just impacts from combat will still wear on the user. It probably won’t result in critical injuries, but it can be exhausting. Even simply fighting while wearing full plate will be extremely fatiguing. Part of this is because armor will effectively trap body heat, leading to the exhaustion mentioned above. This is part of why you’re less likely to see combatants in full plate wandering around desert environments (if the writers know what they’re doing.)
To a certain extent, it’s more accurate to say impervious armor actually moves damage around, into different kinds. You’re trading blood loss for heat exhaustion, which can be just as lethal. That said, this doesn’t make for entertaining game play. “Fight the guy until he’s too exhausted to move and falls over,” may work as a gimmick, but it’s not a mechanic you’d usually want to build into your core game design. As a result, you’ll almost never see armor in games that will fully mitigate all incoming damage. Usually there’s some upper cap on resistances. Sometimes this is 50%, 80%, at least one point of damage must be inflicted, ect.
That said, there’s nothing inherently wrong with a character who’s an impervious juggernaut. Either as an ally or enemy of your PoV character. The important thing is to remember that they’re not immortal, just very resistant.
All of this can be pretty useful, if you’re tailoring your story carefully. Rules, under the surface, can help you keep track of how badly someone was just hurt, or how close a character is to keeling over. At the same time, it is very important to match your characters’ durability to the genre you’re working with. Games that tend towards ludicrous amounts of HP (regardless of if that’s their actual health, or some kind of mystical ability to avoid suffering harm) won’t get the results you want from a horror story. Just like an epic Sword & Sorcery romp will be seriously hampered if everyone goes down after taking a stray hit.