I look at it this way: trad D&D is a simulation of D&D.
I'm not sure you meant it this way, but when
@clearstream quoted just this part it sort of singled it out and dawned on me that I've been having that feeling since I started playing the game in 4e (a system I still have a great deal of affection for, but that's not related here.) Specifically, I always had this feeling that there was an experience that I was pantomining-- where I was always trying to get closer to the ideal of the game,and I could never quite achieve it, until recently.
I agree with this statement, in the sense that what trad DND does, is set a story in a milieu of the fantasy world of DND, but does so with the lens of a movie, book, or video game, where the emphasis is on a structured plot that setting serves. I eventually realized that what I was feeling, was the sense that the world I was presenting was fake, like a facsimile of what it should be where its fake and entirely in service of the main plot and the desires of the players. It was almost like a Call of Duty level, where it was a sequential array of setpieces that emulated aspects of dungeon delving (fighting undead in a crypt, a puzzle room, a boss battle, a negotiation with kobolds) but much like how that Call of Duty city is a series of corridors with all of the other roads barricaded by the game, my games were only concerned with presenting the plot as a movie. When we traveled, it was entirely up to the narrative how fast we got anywhere. When I did treasure, it was entirely to give the players the cool stuff they wanted, and they always ran into their backstories, Critical Role style (this was before Critical Role of course.)
It frustrated me, because I realized, that the cool stuff I wanted wasn't really happening, the world of the game I was presenting didn't feel like a space the players could interact with and make their own decisions about. It was so different from... the ideal that I had about what delving dungeons, and being adventurers in a fantasy world should feel like. But then I had to try and trace that back to work out where I'd even gotten my ideas, and I realized that it mostly came from the fiction that the rulebooks harkened back to-- it came from the implications hanging around the main plots of all fantasy books I'd read, or from the games I played, from the idea that what I really wanted to was to step inside them, and step away from the save the world plot to enjoy and explore the world as a place that exists beyond Chekhov's concerns, in other words, where setting provides a rich playground for the characters to
exist in and explore.
The 'real' DND, at least the one I found, was about finding narrative divorced from plot by presenting a world without narrative assumptions and letting the story be 'the things that happened when we went adventuring, and the people we became' rather than the structured plots that have become ubiquitous with storytelling. Trad DND then to me, is a literal simulation of the world, diverted to the needs of what I almost want to describe as a hollywood plot line-- they're not adventurers first and foremost, they're the handful of adventurers that got picked out by destiny to save the world.
In that context, disclaiming narrative decision making to game mechanics, or even
to myself in a different mindset helps me to create that separation from the needs of the plot-- I pride myself on creating dungeons without thinking about the specific group going through it, with the goal that different suites of capabilities mean different paths through the content that I'm producing, different discoveries, and frustrations. This creates a play space rife with opportunity to have fun, and find interesting things, but that doesn't become smaller by warping itself to the PCs and their plot, which is my ultimate goal. To make them feel like a small part of a big world, and have their stories play out with emergently against that back drop.
None of this is a criticism of trad DND, or not a generic one anyway, its colored by my perspective, and I've run fun plot centric campaigns in the past too.