I don't like to factor it in too greatly to the decline of rock music in particular, because there have still been very big stars in other genres of popular music during the same scope of time. Jay-Z, Kanye, Beyonce, Adele, Katy Perry, Taylor Swift - just to name some - might operate in a world of selling records that is much smaller than it used to be, but they still sell a lot of them. They also still make tons of money. They still perform at very large venues. Their existences as mega-stars is not fundamentally different from the mega-stars of the 80s and 90s.
There isn't a parallel in rock to that. Also, looking purely at the music, it should be apparent to just about anyone that rock stalled creatively. Either because there's nothing left to do or there's nothing interesting-yet-accessible left to do, there's nothing going on with it. Some of the more popular indie bands kept adding non-rock influences to their sound to the point that the music was no longer recognizable as such - the whole twee-pop thing, for example. New bands that adhere too much to an existing style - right now, anyway - are pretty much dead on arrival.
Not saying that there hasn't been good music, just that I would agree that rock as a genre looks like a spent force, as far as being a growing, evolving thing, and pop-cultural touchstone. The baton got passed.