Is there any evidence that a large number of tables are "de-levelling" published material? That they are playing PCs vs Dukes of Hell but using the stats/mechanics for 5th level PCs?Sure... but isn't the point of this thread asking why more people don't play high-level games, not why people are not choosing to play the game WotC/TSR published in the manner they did? To which one of the answers is that we just don't need to play high-level games. There is nothing high-level games have that can't also be had with lower-level games via the use of house-ruling and adjustment (regardless of what or how WotC publishes their material.)
So if it would normally take a table 2 years to reach high-level and then use the high-level material that WotC publishes for it... or tables can use that material a year-and-a-half earlier by just de-leveling it... this could explain why tables aren't bothering to go for or ever reach high-level games.
I mean, I've never encountered evidence of that that I'm aware of.
To me, it seems that most people are simply not using those themes and tropes in their D&D play.