That's also the time frame when MOST people I know became disinterest / disenchanted with the new music coming out, and reverted to listening to older material instead.
I'm not saying the ease of "pirating" music with digital tools doesn't contribute to loss of music sales. It MAY (but the ease of BUYING tracks has exponentially increased too, as well as a reduction to nearly zero in costs of distribution to people -- so I'm not sure).
But quite frankly, we've regularly witnessed trends in popular music that are long overdue, here in the 2000's. As just a random few I can think of off the top of my head? We had the "rise of the alternative girl bands" (Bjork, Sarah McLaughlin, Poe, Fiona Apple, PJ Harvey, Mazzy Star, etc. etc.) in the 90's. We had the brief burst in popularity of ska and neo-swing type music (Mighty Mighty Bosstones, Cherry Poppin' Daddies, etc.). Obviously, we had the huge effect of the Seattle grunge scene. Before that, we saw a rise in popularity of "modern country" and line-dancing, the era of Heavy Metal in the 80's, and a period where rock/rap fusion was popular. So what's really happened along these lines in the 2000-2014 time period?