If there were more support for low-magic games, then the "big tent" approach would be perfect. But as it is, the fact that so many settings claim to be "low magic" or "gritty" or whatever tends to cause more harm than good. People see settings like that and think "See, it's perfectly valid to play D&D without any magic items. We're going to do that," and then are surprised when the party can only take on CR -8 challenges. D&D is not a gritty game, and certain classes (especially the non inherently magical ones) do not function without magic items.
That being said, if designers of low-magic worlds put more effort in to adapting the system to their setting, it would work much better. For example, if a low- or no-magic world flat out said "there are no spellcasting or manifesting or meldshaping characters in this world, period, DON'T ADD THEM IN," then took out all monsters with SLAs, then adjusted monsters' ACs and attack bonuses to match the (now much lower) abilities of the party, then did a bunch of other work...
It would probably not turn out horribly.
But clearly since there's so much work to be done, and so much that isn't compatible with "normal" D&D any more, most designers aren't going to do that. At that point, it might actually be less work to just design your own system from scratch, especially since that would have the added bonus of being tailor-made to the type of game you were trying to play.
BUT, I do think that the variety of settings is a huge advantage for D&D. Even if certain settings don't always work out as well as they were supposed to, just the fact that you can adapt the rules to fit any setting is a huge selling point. In fact, that's one of the things I'd like to see in 5e - less emphasis on new rules (because it tends to introduce power creep), and more emphasis on alternate setting books and the like.