I feel like I almost agree with most of that, because it should be true -- but I don't think the designers of D&D's mundane classes have ever given much thought to realism, beyond not giving Fighters magic powers, or to non-magical settings.
I don't know how much thought the designers of D&D's mundane classes have given to realism or even to what they did give thought. I do know however that early on in D&D's history there was an assumption that D&D's classes could be used successfully to model a great many settings, including non-magical real world ones. This is evident IMO by reading early Dragon magazine, or by examining the 1st DMG, or what is known about Gygax's actual campaigns (Robilar, for example, carried pistols).
Using D&D as the basis for a non-magical swashbuckling or gun-fighting setting just doesn't work without everyone agreeing to make it work.
I would argue that this is true regardless of rules set.
The problem is that there are many, many way to define someone as heroic, in the action-movie sense, and D&D's definition revolves around taking many, many hard shots to go down. If we were designing a Hollywood-western RPG, for instance, that wouldn't be our chief heroic trait.
Which might be true, but I'm pretty sure I do know the reason that the designers choose to model heroism as being able to take many many hard shorts to go down anyway, and that is that they realized that any test based mechanic like, "Ordinary city-slickers might have a 1-in-20 chance of hitting, while he'd have a 19-in 20 chance of hitting.", would still leave the hero with a very very real chance of going down to any ordinary city-slicker. By leaving in the narrative/power of plot protection of a large number of hit points, any bad luck in the game could be translated into a minor 'flesh wound' in the story - leaving the hero on his feet to win the vast majority of such fights.
And it is precisely because this logic is so sound that the 'hit point' mechanic rather than some sort of pure 'test' mechanic remains the most common one in gaming, and in computer gaming in particular.
If you didn't know how D&D worked, but you knew that a 4th-level character was supposed to be a hero, you wouldn't immediately define him as able to take four times as many bullets, +20% accuracy, etc.
Yes, but I would press the argument
that after extensive play testing, you would. Besides which, it's not unusual for the D&D hero to have a 19 in 20 chance of hitting the mook, while the mook has but a 1 in 20 chance of hitting the hero. This is part of how D&D defines heroic in addition to implementing hit points as a form of narrative control.
In fact, the defining trait of a hero, across many genres, is bravery. Bravery stands out to me as (a) lacking in D&D, for the most part, and (b) profoundly important in both real and fictional combat. In a gun-fight, the difference between the "high-level" and "low-level" characters is that the true gun-fighters can and will keep their nerve enough to hold their gun steady, to focus on the front sight, and to smoothly control the trigger, rather than jerk the trigger with the gun poking out from behind cover.
I fully agree, but note that 'bravery' stands out as a trait that is irrelevant to fictional heroic protagonists as well. Fictional heroic protagonists don't have to make 'bravery checks', and if we implemented 'bravery checks' they would fail for the same sort of reason as pure test mechanics fail - luck would overly define the narrative. Rather than having hero that is always brave, we'd have one with a 1 in 20 chance of fleeing from the mook. If you go back to D&D's roots, you can see this in the form of morale checks that monsters have to make, but which PC's are immune to. And again, there are other good reasons for not having 'bravery checks', because as much as possible we want to avoid taking control of the character away from the player.
In my DMing experience, I've found that bravery still remains very important to a D&D game even without a 'bravery check' mechanic, because very often the players themselves lose their nerve, or panic, or decide to throw down their shield and hope the monster eats the slower comrades first, and with often disasterous consequences.