Dumbing your game down does have diminishing returns and Bethesda has probably passed that point.
Skyrim does hit that sweetspot for autistic grindy sandbox, that's for sure. It's got that basic gameplay loop of explore->level up->loot->craft->sell to level up all skills and gain direct xp levels from (in a simplified mechanic even compared to the previous TES games), while developing a stable exponential power progression if you make use of smithing+enchanting, and offering all those radiant quests to not let you run short of dungeons. For people who don't look past the greater picture of it, it will be successful attention catcher, working off the same basic psychology as the majority of the most famous mobile games.
"simplifying it for broad appeal" is cutting it short - Skyrim is in many ways deliberately planned to be the way it is, with changes that differ significantly from simple dumbing down. The end results do end up mostly showing off the simplification, sure, but it's not fair to only lay it down to the generic argument of "it's simpler, therefore it's more popular". It's a fallacious interpretation of the Lowest Common Denominator appeal. If Skyrim was already sufficiently simple to broadly hit the target audience of people who like "games where you fight things with swords and magic", dumbing it down further will just reduce the amount of possible things for people to spend time in, therefore be less interesting and shorter-lived.