Why?
You not only are showing a strong bias here, but you don't support your view. And you also seem to be dismissive of anyone with a different view.
I too love making homebrew worlds and campaigns. And I think when I do my players have a noticeably more enjoyable and rewarding time.
BUT, to make that the preferred or standard expectation, or implying that a GM that doesn't homebrew is... not doing there job is ... detrimental to the hobby.
Many GMs don't have the time, the ability, or the confidence to create or run homebrew. Failure to provide a complete game, which includes a setting, will severely limited the growth of the customer base.
IMO, for those GM's that do create their own homebrew, converting a module from one setting to your own isn't that big a deal. And as have been pointed out, there is really no way to make a generic or setting-less adventure because there are assumptions built into the core game.
Maybe I worded things poorly.... I think it's fine for GMs to use published settings. Currently, I'm using Eberron, mainly because of time constraints, though I'm not sure I'm getting much benefit in that department.
My objection is really that I'm concerned that Forgotten Realms is becoming (has become?) synonymous with D&D. IMO, this is an extremely bad thing and should be actively avoided. This is not just because it's the Realms. I'd be unhappy if Greyhawk or Eberron became synonymous with D&D, too. IMO, one of the defining characteristics of D&D is that it is not tied to a specific setting. It's not completely devoid of all flavor, of course: you have chromatic and metallic dragons, three goblinoid races, etc. But those are scaffolding for the GM to work with, rather than constraints. Mind flayers
might come from the Far Realms or they
might come from an ancient wizard's experiments. Heck, humans might come from an ancient illithid's experiment to create a tastier elf.
It's like working with my kids on art projects. Sometimes, they end up with something that's actually really beautiful. They show it to me and I tell them it's great. Then, they decide to add more to it until the whole thing is overly busy and, while each element is well done and attractive, the whole is actually no longer appealing. Other times, the youngest gets in a "blue mood" such that the grass is a shade of blue-green, the sky is light blue, the house is slate blue, and so forth. I love blue. Monochromatic isn't always the best choice.
Looking at 5E, the designers can't seem to leave well enough alone, sometimes. The cardinal example is the Mystic. Mearls actually put together a really nice chassis for the class. I don't happen to object to the idea that some settings might use the Far Realms as the source of psionic power, but his fluff strongly implied that such was the default, rather than just an idea.
I can't actually recall having a serious negative reaction, when the PHB came out. I'm sure I did an eye roll at the amount of space devoted to the human ethnicities of Faerun and was probably mildly annoyed at the Weave sidebar, but no big deal. The appendices are really quite good and balanced as are some sidebars (Monastic Orders). The LMoP adventure used the Realms, but that was really just a small map of a remote area. I was really irritated that HotDQ wasn't better identified as being pretty tightly coupled to the Realms (I tried to rework it; it almost requires scrapping and starting from a story kernel and the story wasn't actually that great). PotA was a pretty good adventure where the setting was largely irrelevant, once you got into it -- I would have preferred to have the various faction hooks stuck in an appendix next to the setting conversion material, but the FR maps is no more intrusive than LMoP. OOtA and SKT both suffered from being tightly coupled to the Realms. CoS was mostly fine, but was a missed opportunity to put the faction material in an appendix or web supplement with similar launching points for at least a few other settings.
So, there are a couple things that I'd like to see done differently.
1) Clearly label adventures that are tightly coupled to the Realms, somehow. Yes, you
can convert them, but some just take too much effort. I just want to know without having to read reviews or forums.
2) For "generic" adventures, go ahead and use Realms maps as the default, along with the names for gods, etc. Just leave things like pages 14-15 of PotA and 21-22 of CoS (Faction-specific hooks) out of the main body of the adventure. Put them in an appendix or web enhancement.
3) Continue using something like Appendix C of PotA. It wasn't perfect (by far), but it at least paid lip service to the other worlds. As a corollary to this, produce more adventures that aren't tied to the Realms.
4) Publish a couple of adventures that are tied to other worlds (Eberron would be my preference, but I'm open to options). When you do so, really do it. Have no more reference to the Realms in the Eberron adventure than you put Eberron references in SKT.
5) Put out a SCAG-like book for at least one other setting. In truth, I was fine with the phantom support for settings. I'd be okay with, say, a Dragonlance-like approach to another world where the adventure runs deep enough that a separate setting book isn't really necessary. But, actually
do it and do it well.
Looking at that list, it was a long way to circle back to just being tired of the Realms being at the center of things. I don't mind some of the references; the Realms are part of the D&D canon. I don't mind the Realms getting a couple tightly-coupled adventures or the SCAG. I'm just ready to move on.
I've actually said, multiple times, that I'd probably just go with the flow of published adventures, if they actually did rotate through worlds. I like the idea of Athas, but really don't think I want to commit to playing it long enough to buy a bunch of setting material. I'm definitely in for a single hard-cover adventure, though. Ditto for Ravenloft, actually -- maybe a couple trips, here. I'd dip my toe into Birthright, too. Planescape has never appealed to me, but I'd try a one-and-done. As much as I dislike the Realms, I'd even be up for cycling through it, if that's what it was -- a cycle. I could park it in Eberron for a while, but I'd be willing to move on from that, if I got a nice, flavorful adventure that let me feel like I'd experienced the setting well.
As an additional benefit, cycling through the worlds implicitly looses any subconscious fetters on rolling your own. It provides the encouragement I'd like to see for home brewers.