I don't overall mind D&D's dominance, however, I wish it was significantly more modular in the core rules as was promised instead of relying on miscellaneous 3rd party creators to fill in the blanks (I'd like to minimize the amount of resources I have to parse and read through to run the game like I want).
As a GM type, I do want to play loads of other games, but I might be able to satisfy those wants and needs more if D&D supported them better as written.
I find that the significant sales of games like Dark Souls and Elden Ring does mean that the demand for tougher games with higher lethality exists, and D&D could fill the gap on both ends - low complexity and super-heroics, but offering modular rules that can run with higher complexity and lethality. Unfortunately, D&D has over time distilled into what Hasbro/WotC thinks the "purest", lowest-common denominator form of D&D is, without considering all that's been lost in that distillation, and disregards a desire for other styles of D&D.