Hi, all--
The 1999 study is well known mostly because its results were shared, in part, with the public, and also because it was the first act of real, professional market research ever carried out for D&D. But it's a drop in the bucket for WotC, which has, since then (at least through a year ago, when I was running D&D), continuously conducted formal and informal market research to best understand what gamers want out of D&D.
Among other things, WotC conducts (or, again, did while I was there--and probably still does) an annual awareness and usage study, an ongoing consumer panel, focus groups for key products (generally things in which the learning process is key, such as the Basic Game or minis starter set), and surveys and interviews at big shows such as Gen Con. There's also market research done on the retail side of things, including a monthly study of retailer purchases and activities across the entire hobby gaming category.
WotC has professional market researchers on staff, and employs outside agencies. Regardless of our speculation about the validity of sample sizes, the techniques used are up to industry standards for this type of research.
Market research is just a tool, of course. Its results can be misinterpreted, ignored, or poorly acted upon. Even under the best of circumstances, market research is just one factor contributing to a good business or creative decision. The people who run D&D may or may not make great decisions, but either way they have a substantial amount of continuously-updated market research to call upon.