I don't see anybody claiming geekdom as a whole is in a decline. The discussion is about D&D and role-playing games.
The heart of the discussion is about RPGs, but Hussar and I got into a side trek on genre films. I think in hindsight he was right that science fiction and comic book movies are aplenty today (now that I have reflected on it), but I do think standard Fantasy fair is less plentiful than in the early 80s (at least wide release stuff).
However, you are right, the question is whether RPGs are on the decline. I think it is a hard issue to answer because there isn't much data to draw on. Even sales can be misleading as a devoted core audience can drive sales (and people can play games without buying them---not everyone owns monopoly but most people have played it even if they don't).
My belief is we've lost blood over the last ten years and the products I see look a lot more like "by gamers, for gamers". That could be good or bad depending on your point of view. On the other hand, I don't think we've ever had this much gaming material before. Everyone and his brother is self publishing these days. So chances are you can find a game for any concept that appeals to you. Though I think most of these companies are labors of love and not very profitable (or just making enough to keep producing books).
Again this is all perception. When I was a kid in the early 80s, it felt like I was surrounded by a gaming boom. I just saw D&D everywhere, and I didn't play until '86. In the 90s, it felt like we had another boom (probably smaller) with WoD and the endless clusters of TSR settings (yes TSR mismanaged and went out of business, but people were playing these settings when they intially came out). When TSR went bust I noticed the number of active games in my area declines (I think people just have a need for their game to be actively supported). Then when 3E came out (which coincided with the LOtR and Harry Potter films) I think there was another huge boom (probably close to or bigger than the early 80s). But from where I am standing now, it looks like things are on the decline again.
So I am not saying the hobby is dying. Nor am I someone who wants to go back to 1982 (though I do believe WOTC could learn a lesson about modules from the 80s-90s). In fact, if I had to pick a year I would least want to revisit, it would probably be 1982. I just think there are booms and busts in gaming, and I think the gaming culture has codified itself more overtime (which could have an impact on people trying to break into the hobby). Either way I think as gamers one thing we can do to grow our hobby (whether it is on the rise or falling) is game openly to reduce the stigma (for example playing at the mall foodcourt, coffee shop or bar) and be more open to new gamers (I think this last part is also dependent on game publishers making RPGs more understandable to the new gamer). Note: Not saying gamers are unfriendly to new people, I just think we sometimes could be more proactive (I know I could at least).