I both agree and disagree with you here. On one hand, if a demographic is significant, it should not be exlcluded just because it is difficult. GOOD Market Research does not deliberately over-represent or exclude anyone. If you are trying to measure the popularity of D&D among the general population as a whole (which would be a wholly depressing project) then you let in young players, old players, Presbytarian Ministers, EVERYONE...to ensure accureate sampling.
But on the other hand, Garnfellow raises a very good point. Though I am grossly oversimplyfying marketing in saying this -- there are two distinct goals of any marketing campaigning.
1) GROWING your market to include more people.
2) INTENSIFYING your current market to buy more products.
Now think about WTOC's goals when they implemented their 1999-2000 campaign (and, I would guess all their subsequent research) If you're mainly interested in people who just PLAY D&D, then you need to find a way to target your research to a very specific sub-population...be it through magazine subscriptions, websites, feedback post cards...to name just a few tools.
Note However that this post is about MARKET research. If I was contracted to do Market Research for WOTC, I wouldn't be sweating too much about who is PLAYING D&D (except in a background/sector context kind of analysis), I want to know what is important to people who are either ALREADY BUYING D&D PRODUCTS or are in demographics that are LIKELY TO EXPERIEMENT WITH BUYING D&D PRODUCTS.
In this case an eleven year latching on to an older game group, and a 30-year grognard who has not purchased any new products since 1983 have something in common. While, in both cases their contribution to the hobby is commendable on the other hand, their views on product marketability are close to inconsequential.