When I say “RPG material,” I mean things like statblocks, classes, etc.
I don’t think I’d call D&D a consistently great base to start from, honestly. For certain settings and genres, yeah, but others are seriously harmed by trying to make it fit a fantasy adventure system. Mysteries and heists are forced to include a lot more gratuitous fight scenes just to balance all playstyles, for example.
Similarly, I worry that using d20 as a one-size-fits-all will reinforce comfort zones than expand them. Why would a person play the Star Wars FFG system (which I prefer) when they can play the Star Wars d20 system instead and not have to learn new rules? And if you’re making a new system, then you clearly need to include six core stats, because RPGs have six core stats in them, right?
D&D isn’t bad, nor is homebrewing D&D for a new setting. D&D as an assumed foundation of all RPGs can be a serious issue.