It would have to be opposed rolls, since every attack is not equally well placed, and that's dependent on the attacker's skill / luck. Defenders rolling dex saves is a perfect way to implement DR.
Any DR system rolled out should modify any and all subsystems to make HP = actual penetrating wounds. Realism module, FTW.
But, this mechanic as it stands, means the default game does not support simulationists at all, where it clearly once did (one could ignore the nonsensical definition of HP, better that than try to distort the meanings of all ancillary words used to describe discrete game events as being subservient to game definitions, that's awful).
In a fast & loose game, I still want the narration to be coherent. This prevents that, entirely. Miss is not "miss", it's still the opposite. These words are defined as their opposite, everyone knows that. Once you start second guessing every single word, there is no coherent way to imagine what's going on. And that's bad for a game based on imagination. You can say certain things are vague, but "you succeeded" and "you failed" are two mutually exclusive states that should not have overlapping truth values (cannot, really). Only in quantum mechanics do those weird things happen, but D&D is not quantum mechanics, and shouldn't be as counter-intuitive to play. I can explain the Heisenberg's Uncertainty Principle or how special relativity means reference frames are relative (but the speed of light isn't!), but I don't want to have to live in a quantum superposition of states just to play D&D. Most people can't even pass basic physics questions, let alone ascend into higher end physics topics, and I have no desire to play a game where I'm forced to have to say "that's nonsense" every single round.
D&D is inconsistent in a lot of places, but this is round-by-round, in your face, can't deny it, can't escape it, pure stupidity. HP is meaningless with the definition they gave. There's nothing magical about fighters. There's nothing about luck being lost when you lose HP (your odds of success at any contest are unchanged from 1 HP to max HP).
In short, HP is defined to be meaningless sludge, despite it having the term Hit right there, front and center, people think misses are equivalent to hits in D&D, sort of like how the Ministry of Truth teaches BlackWhite and DouplePlusUngood NewSpeak to its subjects, as a form of mental torture and control. No, thanks, I want RPGs to make at least some sense, if not all the time, at least most of the time. Fighters attacks, or character health has no divine or magical agency narrated ANYWHERE in the game, if they wanted to they could easily say D&D PCs are all magical, and HP is gaia's Love score. But it isn't. D&D allows you to pretend to be a joe bloe, off the street, picks up a sword and fights in a world full of magic. But he himself isn't magical. Why? Because it's defined that way! Fighters are non-magical classes. So are rogues. Magic effects belong in magic classes. Never missing is a magical effect, and belongs, if anywhere, in classes specifically defined to be magical in nature.
People love quoting that HP definition, not realizing how easily and utterly demolished it can be. Trivially. HP has never been coherent with the way combat is narrated, from the start. Combat is narrated like wounds are accrued gradually until you die, yes they ignore delibitating wounds, or slowing you down, but that stuff is handwaved out of the game because even for simulationists, it's a bridge too far and not worth the effort. Combat is specifically talking about sword thrusts and cuts, it's absurd to say hits and misses don't result in injuries, absurd on its face! Try getting hit from a sword and not bleeding. HP, as defined now, means you CANNOT describe hits as being different than misses, because misses with a weapon now can cause damage and even kill you. How? Doesn't explain. Just handwave, say it's "abstract" so false stuff is true, refer to nonsensical definitions instead of trying to clean them up.
New editions should clean up bugs from prior ones, not introduce new ones. It's totally going backwards. If the game is to evolve, we have to identify clearly what bugs there are, maybe perhaps not solve them all in a single iteration, but over time the game should be perfected, instead of remain as nonsensical as the day the definition of Hit Points was penned to a page. They could solve the HP-as-meaningless stat problem, but chose not to! Why? Specifically to allow impossible mechanics to exist.
It's funny calling GWF narrativist, because you can't actually narrate it without uttering nonsense. Gamist, is better. But it means they've given up any pretense of being inclusive to those who want the game to make a semblance of physical sense with this. Not only physical, but logical. No human is so perfect that they cannot fail to injure a foe, under any circumstance, with their sword. No one. Without magic. But since fighters aren't magic, they shouldn't get this.
Warlocks? Sure! Give it to them! Just call it a magical ability and be honest about it. I'm not buying it, and I've worked on MMOs with flying dragons and respawning and zones and camping and so on. I can handle abstract fine. I can't handle utter nonsense BS garbage. And I won't pay for a company to insult my intelligence like this. Just admit it's magical, and remove it from the fighter.
I want the fighter class to be simple, straightforward, easy to play and easy to narrate. Some people don't, but that's their problem, D&D fighters have always, until 4th ed, been easy to understand how they do their schtick. This is a step back into the abyss of bad game design that will drive people away. Not only simulationists, but anyone who has the remotest respect for the english language or basic understanding of physics, or logic.