earlier editions both expected and encouraged the use of PC rules to describe NPCs to various degrees
This is true of 4e too. For instance, a monster or NPC stat block looks very similar to a PC one (though if the PC is much above 1st level the monster/NPC one is probably quite a bit simpler), and the same action resolution rules (with some minor exceptions around action point usage and what happens at 0 hp) that are used to determine how a PC affects an NPC/monster in combat are used to determine how an NPC/monster affects a PC in combat.
It's true that 4e's NPC/monster build rules are different from those for PCs, but this is true in AD&D as well.
I was reminded of this recently when reading the PHB errata in Dragon 35 (from 1980), p 34:
The limitations given for races on the Ability Score Table are intended to apply to the entire race, not lust player characters. Therefore, several inconsistencies between the maximum scores and the player character racial adjustments are not mistakes. For example, although player character elves have a -1 on their constitution, the racial maximum for constitution of elves is 18. This is because, as explained in the Dungeon Master’s Guide, exceptional non-player elves may have a constitution of 18. Likewise, although it is impossible for a player character halfling to roll the maximum 19 constitution, a non-player halfling could, due to the bonus received on constitution for NPC halflings in the DMG.
Page 100 of the DMG, which sets out those racial adjustments for NPCs, also sets out stat adjustments and requirements for NPC members of character classes. They are not the same as those for PCs (eg an NPC fighter, rather than needing STR 9 and CON 7 as a PC does, gains a +2 to rolled STR and a +1 to rolled CON; an NPC monk rather than needing 15 STR, DEX and WIS, 11 CON and 6 INT and CHA, as a PC one does, needs only 12 STR and 15 WIS and DEX).
Fast healing also means that you can't find a seriously injured NPC anywhere, which invalidates any number of possible narratives. (Or it means that you have different rules for PCs and NPCs, which causes all sorts of other issues.)
fast natural healing plays to a very specific interpretation of hit points - one without a significant physical component - making any narrative that would include such things entirely inappropriate.
We can't play a story where someone is waylaid by a week due to injury in a world where all damage disappears overnight. We can't play a story where anyone could be waylaid for a week, because any significant physical damage would be inconsistent with overnight or martial healing.
Who is within the scope of "anyone"?
I GM a 4e game, and I have had NPCs who have suffered severe physical damage - for instance blindness and maiming from being in melee with hobgoblins. The hit point and healing rules didn't pose any obstacle to this (because they are not a system for modelling or deciding what happens when NPC orcs fight NPC hobgoblins off-screen - I just make that stuff up!).
Even when it comes to PCs, many 4e players have had PCs waylaid by a week due to injury by the simple expedient of changing the extend rest rules. The only reason this seems not to work for the 5e second wind is because (unlike it's 4e counterpart) it appears to have no resource cost. But I wouldn't be surprised if there is some other part of the game (eg in the rest rules, or at least some variants of them) that in fact impose such a cost.
I'll assume that I'm supposed to be looking at the part where it says "broken leg"? The thing is, I have no idea what that means. It's certainly not something I've ever seen in any of the rule books. What's the mechanical effect of it? How long does it take to heal?
Because suddenly we're using a model that keeps track of specific injuries and may or may not be de-coupled entirely from the Hit Point system. That's far from simple. I mean, not having to track specific injuries like that has always been one of the strengths of abstract Hit Points.
I'm not sure how serious you are here.
In my own game, where one of the NPCs had a broken leg, what were the mechanical effects? It meant that the character in question couldn't walk, and needed to be carried. (From memory, one of the PCs healed the break using a Remove Affliction ritual.) A blinded NPC can't see. Etc.
If you're saying that abstract hit points handle all this in a way the free descriptions like "broken leg" don't, I'm not sure I follow. In a world in which no injuries ever occur other than hit point loss, there aren't any broken legs or any missing eyes either, because losing hit points doesn't impede movement (which is a definite consequence of having your leg broken) nor impede your vision (which is a definite consequence of being blinded by a sword-stroke across the face). D&D has never had any mechanic, other than extremely magical swords (vorpal, sharpness) and staves (withering), for imposing serious injury by way of fighting with weapons. Certainly lost hp aren't such, because they don't impede you and can be naturally healed quite easily - whereas broken legs don't heal in a week, and humans never grow back lost limbs or eyes.