Neonchameleon
Legend
I reject the premise. Even if we take Google search volume as a proxy for popularity--and I'm okay with that--those graphs don't show a game in decline!
Check out this graph, which shows 2007 to the present: http://www.google.com/trends/explore#q=dungeons%20and%20dragons%2C%20d%26d&date=1%2F2007%2012m&cmpt=q
It's basically flat.
So what you've really proved is that D&D /used/ to be in serious decline, but it's been basically flat for the last 6 years. Not such a big deal.
edit: oops, the link I posted is for just 2007. Still, things have been basically flat for the last four or five years.
Huh, fascinating. Which means that 3.5 almost destroyed D&D - and 4e stopped the rot. Not at all the narrative common in many quarters.