As a huge GH (but really Rock Band 1/2, which hit the sweet spot for me) fan, thanks! If your open to answering questions, I want to know (in your opinion) why the genre's appeal has more or less worn off now?
Our hope at Harmonix was that before that curve of interest fell off, we'd latch onto another curve by making Rock Band be a core way that people experienced rock music in general; you would expect that your favorite group would release a Rock Band version of their new single, and they'd expect that they'd have to do it to satisfy their fans, and it would become sort of self-sustaining. That was one big idea behind Rock Band Network. In retrospect it failed to accomplish that, but I think it was a really good try to keep the genre sustainable for more than a few years.
Note that we still make money from selling downloadable content; there are thousands of downloadable songs available, and of course sales have gone down some since the good old days, but they're still a money-maker. So people are still playing it.