Really, I've found D&D has always been a fairly good generic fantasy system meant to be modified (or was anyway). Especially when you consider that the rules are only there to fall back on for when the Braunstein game of "What does your character do?" fail. But still, I've always ascribed to that the best GM running the worst system will always be better than the worst GM running the best system, and D&D is the game everybody knows and is most likely to be willing to play.
Completely agree. The idea that "D&D doesn't do generic fantasy very well" has always struck me as one of those instances where the speaker is saying more about themselves than about D&D. In more than three decades of playing multiple editions of the game, I've found that it, in fact,
does do generic fantasy—and a lot of other kinds of fantasy—quite well. You just need to be willing to modify the system, and be adept at doing so. It's not enough to simply recognize that D&D is a toolbox; you have to know what the tools are, what each of them is good for, and which ones you'll need for your current campaign.
Insofar as "but if you change it, it's not really D&D" goes, I think that misses the point, since changing it
is central to the D&D experience, whether it's things that you alter for your home campaign or a setting that makes changes right out of the (literal) box.
Likewise, the idea of "just play a bespoke system instead of hacking D&D" similarly seems to miss a lot of relevant points. For one thing, it's usually not realistic to expect your entire group to learn a new system—purchasing new books and sinking hours of time into reading them—just to get an experience that's probably going to end up mostly the same as if they'd just said "let's cap level advancement at 10, and stick to the PHB only, but no warlocks, dragonborn, or tieflings."
And really, that's the issue; that learning a new system quite often costs more than the payoff you get from it. Maybe that bespoke system
does facilitate a better play experience; okay, but
how much better? Enough to justify going to all of the time, effort, and trouble of learning it? Certainly, for some people that's not really a factor; they have the money to buy the books, the free time to read them, and the energy to retain all of that. But I know a lot of adults for whom the stress of daily life would make that feel like a homework assignment. It's not worth it for them to re-learn character creation, the combat system, character advancement, and a hundred other tweaks scattered throughout the rules. Getting 90% of the play experience you want for zero effort (since you've already learned D&D) is more worthwhile to those people than getting 100% of the play experience after considerable effort.
Which presumes that the bespoke system they invested in even gives them the better play experience that they were promised.
Far better to simply modify the game you already know than to sink your time, money, and energy into what might very well turn out to be a fantasy heartbreaker.