There is a possibility that D&D can be sold as a "classic" game, with only the core rulebooks on sale and no updates, ever. No supplemental support is needed. Although D&D has never been sold in that manner, I can see it becoming that way in the future.
I've seen many statements from those in control of D&D about how small the proportion of gamers is that actually buy supplemental books and adventures is. That's mainly because the core books provide all most people need for years of fun. When someone says, "I don't want Frostburn - it doesn't offer anything to me that I need", this is actually pretty normal. It doesn't bother me in the slightest. People like myself, who really enjoy new D&D material (almost no matter its nature!) aren't so common.
(We do exist, though, which is why I get annoyed by the people who say "Frostburn sucks! They shouldn't have made it!")
The main audience for D&D who buy only the core books, and then there are the people who want more optional books. A certain respectable profit can be made out of just selling the core books; various game designers can make a living out of the optional books.
When the audience for the optional books drops below the point at which they are profitable, then D&D moves closer to becoming a "classic" game - by which I mean it is not further developed, only the core rulebooks are printed - and possibly a couple of evergreen supplements.
Now, a new edition might revitalise the market for a while, but such things are tricky. D&D supplements work at present, but it's not assured they'll work in the future.
Incidentally, D&D Miniatures seek to tap into another group of D&D players (as they are accessories, not supplements), though their purpose is confused by also being advertisements for the Latest, Greatest supplements. When you see an Eberron miniature, it is a pointer to have a look at the Eberron campaign setting - in addition to being a miniature you might be able to use.
Because of the wide variety of tastes amongst D&D players, the overall picture is somewhat confused. Wizards are currently trying to provide different products for different groups of people, whilst hoping that the groups are big enough (or overlap enough) to keep the supplements profitable.
This is somewhat different than the 2e approach, which had the effect of artificially splitting the market through many competing campaign settings. The end result of Wizards' strategy could be the same, of course.