4E doesn't have an ongoing, supported setting.
A counterpoint is that original D&D and AD&D didn't have an ongoing, supported setting until after 1987, by which time I believe they were already in decline from their sales peak. (1982-5)
Greyhawk was a setting "by default" more in the early days than something that was strongly supported setting, though it was just beginning to move out of that obscurity when Gygax left TSR and "good" support of the setting ceased for a number of years. Dragonlance wasn't really an ongoing setting (and has had many problems in later years because the initial release really told the story of the setting). It was only with the release of the Forgotten Realms in 1987 that you really get to the conception of the supported setting with sourcebooks, adventures and novels appearing.
The trouble here is that the Forgotten Realms, twenty years on, are no longer the sales engine they used to be. Wizards got ample proof of that through the 3E era. The decision for only 3 books per setting isn't something that was decided on just to be different; it was a reaction to the poor sales of ongoing campaign supplements in mid-to-late 3E.
The idea of having a well-selling campaign setting is a good one, but what campaign setting are we talking about? Forgotten Realms is played out, and I'm guessing Eberron - for all its merits - doesn't come close either.
From my perspective, they have actually produced a new campaign setting: Nentir Vale. In a lot of ways, it is more developed than Greyhawk was in 1985. I know that a key component of my enjoyment of 4E derives from the mythology of the Nentir Vale setting, as expressed through the adventure and later 4E supplements.
If we look at Pathfinder, how much of their sales are due to Golarion? How many people are buying their products mainly due to the setting? While I don't buy Pathfinder RPG products, I have been getting the Adventure Paths, and now I'm in my fifth AP with them, I have a hard time believing they're all in the same setting: they're just too diverse.
A popular setting is a welcome bonus, but I don't think it is (a) essential or (b) easy to manufacture.
Cheers!