All I'm looking for are rules updates for the various campaign settings that fans love.
You can't tell me that a rules update wouldn't sell. There are certainly many types of non-"big story" books that will sell. D&D isn't only about the story / adventure path. Campaign settings are also huge part of D&D.
And what pray tell are the "rules updates" these settings need? Eberron has got their rules updates-- they made a UA article for them over a year ago. Greyhawk has no special "rules" that need updating. As far as I know... Mystara, Birthright, same thing. Dark Sun has the rules for defiling which have not yet had someone at WotC cobble together a workaround, I'll give you that-- and the Thri-kreen and Mul could probably see a UA article thrown their way too.
Once you get those couple things though... what game mechanical things do WotC have to write up for people in order to play these settings? What's holding players back from playing those settings right now?