Contrary to genre, at the very least.
I'm big on in-fiction consistency (without it, the game becomes a waste of time), and part of that consistency resides in the PCs being, first and foremost, residents of the game world just like anyone else there (i.e. all the NPCs). They were born here, they grew up here, they have friends here who were born and grew up just the same as they did.
Well, that is a particular school of thought when it comes to fiction. Not usually heroic fantasy fiction, more like slice-of-life, but, well, it's a school of thought that exists.
Sure, the PCs maybe ended up being a cut above in one way or another, much like having a school chum who went on to become a Rhodes scholar - but underneath, that Rhodes scholar is still the same as you; and the PCs are, underneath, much the same as their non-adventuring contemporaries.
In another thread someone (Celebrim?) was going on about people being unable to conceive of what things were like in a period of history more than 100-200 years prior to their own time...
...I wonder if it's not more like 50?
(I mean, I'm like 50, and I remember larger-than-life heroes - OK, and criticism of them - but I didn't think the very idea had be wiped from the very genre within which they were most firmly ensconced.)
Giving logical abilities to generic monsters that they should have had all along (in 1e, for example, by RAW Giants don't get strength bonuses to hit and damage!) is an absolute no-brainer.
I thought 1e was clear that such hypothetical bonuses were just bundled into their more favorable combat matrix and handfuls of damage dice?
The only edition that did it excepting for exceptions was 3e....
I guess sorta back on topic, yes, 3e is the only edition that really went all-in with the PCs & NPCs & Monsters are All The Same Things. Almost to the extent RQ did it.
But, while 3e was say over on that side of the spectrum and 1e & 4e (& now 5e) way over on the other, there was not much "3e isn't really D&D because all NPCs have levels" being bandied about.
And, even 3e had lesser classes meant for NPCs (thought he 'lesser' Warrior, clearly inferior overall, with d8 HD and no bonus feats, had a slightly better skill list than the Fighter - yeah, I'm sorry, still perplexes me).
So, yeah, treatment of PC vs NPCs vs Monsters seems "orthogonal to" the Essence of D&D. They've been treated very differently to virtually the same in different editions that were clearly accepted as Really D&D.