This discussion is interesting, albeit a little too heated.
My own experiences of roleplaying chime with what a few others have said.
I first played RPGs in the early 80s to about 1989. I have never seen as many people playing roleplaying games as then. There were the game books in book stores, rulebooks in normal shops etc. At my school I ran a club where we had 30-40 players, most of whom had heard about Roleplaying before they came to us. Yet I also remember that almost none of these players had any books; I know because we had a collection of books belonging to the club, that people could borrow.
I left RPGs until about 1999 and when I came back to it, I found that players are now a totally different breed. Most have many rulebooks and they are heavily invested in the settings and games and many of them also DM.
In the 1980s, it was almost like there were two classes of people; players and DMs. If you had the rulebooks, you tended to DM because there were so many players who did not. You also tended to have a different mindset in terms of investment. It is perhaps no coincidence that it is mostly the DMs from that era who are still roleplaying now.
So I can believe BOTH the points of view expressed here; i.e. that D&D was MORE popular in the 1980s amongst people and had a much higher penetration but that this was a fad and never translated into hard sales.
I can also believe that, in terms of sales, this is the golden era of gaming because young people have SO much more money than I ever had when I was their age. Indeed I have so much more money than my parents have. Just as an example, how many of you have two PHBs for 4E?
So I would just like to conclude by saying Popularity does not equal sales, especially when you factor in the sea-change that has taken place in the world since the 1980, in terms of disposable income etc.