There's a flip side to all this as well that needs to be taken into consideration. Do these alternative settings actually NEED a full setting?
For example, they borrowed liberally from the Elemental Evil campaign to build Princes of Elemental Evil. Yes, it was a Greyhawk campaign, but, they filed off the serial numbers, gave it a new paint job and poof, now it's a Forgotten Realms campaign. And it works. It's been pretty well received. Those who want to port it back into Greyhawk can certainly do so, although, WotC isn't about to release a Greyhawk version of the SCAG to help you with that.
Take Ravenloft. Now, here's another setting that saw it's genesis in a module. And, I'd hazard a guess that most people are more familiar with the module than with the entire Domains of Dread thing. So, why not do the same thing as Princes? Take a chunk of the Sword Coast, some nice mountainous region without a lot of people, and poof, there's Barovia. It's not like you need this huge area - something like about a fifty mile radius circle around Castle Ravenloft would catch it. The Sword Coast region is BIG. Plunking down the geography would be easy.
So, now what do you need? Well, Sanity rules, Corruption rules, and a handful of specific monsters and some backgrounds. Again, a book the size of SCAG would easily cover this, or, heck, you could cover 90% of that in the Ravenloft module itself. Something along the lines of Return to Castle Ravenloft where you have randomisation mechanics to determine NPC goals and you're pretty much good to go.
Does a Ravenloft inspired AP need to have Lord Soth's Domain of Dread? Or any of the other ones? Are they actually integral to having a gothic horror experience? After all, isn't that what Ravenloft is about? Bringing Gothic Horror to D&D? Whether they do a kind of reboot and rebuild a new Castle Ravenloft style campaign, or maybe use the Sundering to say that the Ravenloft Domain got sucked into Forgotten Realms, it wouldn't be that hard to do either way.
That way you get to leverage the commercial popularity of Forgotten Realms and still bring something like Ravenloft to the fans. I'm looking at that poll on the Front Page of En World that says that there are as many people playing FR as homebrew, which jives with what Perkins was saying earlier. And FR is tripling the numbers of any other setting. You're never going to abandon that to chase those other numbers. It's just not going to happen. Far better to try to entice those other setting fans into FR by doing what FR has always done - steal liberally from every other setting.

I mean, good grief, the most iconic FR elements are almost all stolen straight from other settings. Drow? Yup, Greyhawk. Waterdeep? Greyhawk city. Under mountain? Castle Greyhawk. Tieflings and Aasimar? Planescape. Kara-Tur? Oriental Adventures. On and on. FR has always been the Borg of D&D, assimilating whatever it could suck up.
It makes far more sense, to me, to try to hoover up support from other settings and fold them into Forgotten Realms than to try to chase those smaller groups and hope that enough FR fans will make up the difference.