Yes, D&D's D20 system is so popular (and hence BG3's popularity) because it is very accessible to the vast majority of gamer types. It is very simple (I would say superficial) and easily understood, so gamers can figure out exactly what the numbers mean and what they do. This is in sharp contrast with the system Sawyer created for Pillers of Eternity, which is in my view far superior to D&D mechanics, but it is extremely complex and opaque, and most gamers will have a very difficult time trying to figure out and understand how the numbers work and what they do. This is, again in my view, the key reason for BG3's huge popularity and PoE being very niche. Even I, as much as I love the PoE games, often get frustrated trying to figure out the PoE mechanics.
So yes, there is something to be said for mechanics that are simple and easily understood by most people. My biggest problem with D&D mechanics, though, is not its simplicity but rather that it so very strongly relies on random chance/luck in determining outcomes, so much so that the player's character-build choices don't matter that much. This is what's so great about the PoE mechanics, where random chance plays only a very marginal role in outcomes, and it is your build and level-up choices that ultimately determine outcomes. But then, it is all the more frustrating when the player cannot figure out the meaning of those numbers when making those build choices. So I would love a system that had D&D's simplicy but where random chance only plays a marginal role and it is your build choices that matter most in outcomes.
