Looks to me like CD sales were a bubble, and there's no reason to expect the music industry's success to continue as it has for the prior decade.
The late 90s was also the end of the pre-internet era, where the dominant way to market music was radio, TV, and print. In 1995 you had to try hard to discover music outside the mainstream. In 2005 you could go to MySpace and listen to an hour of music without ever hearing a top 40 song.
So, not only the labels were selling music by new artists at their usual pre-Internet levels, but there was also a big demand for old music in CD format. I suspect many older people that would not usually buy new music ended up buying many of their old albums again.
Obviously the answer was MP3s, but I think they might have missed the boat there...
The problem was that it was too close to their current bubble at the time. These are the kind of things that large businesses try to artificially separate out so that they can maximize profit. Why bring in a new technology now when I can wait until later and make more money in the interim?
However, it is very interesting to not how Vinyl, Cassettes and CDs all dovetail into each other nicely. I would be willing to bet that the rise of mp3 downloads and the decline of CDs would dovetail equally as well.
The problem was, I think, the mp3 wasn't a media format invented by/for the music industry.
If I download music illegally, I can download GBs upon GBs of music which would cost me thousands of dollars were I to purchase it. What if I don't have those thousands of dollars? It's not like you can say that I would have bought all of those albums if it was impossible to pirate and then add the thousands of dollars to 'lost CD/Legal Download Service sales.'
However, I don't think there will ever be any trustworthy comparison like this, as the number of downloaded tracks was never reliably tracked in a manner that would be comparable to the RIAA unit sales data. Even if you could estimate the number of tracks downloaded, it would end up including things the RIAA doesn't include, like mashups, indy tracks, concert bootlegs, simpson quotes, etc.
I have a feeling that (other than the 'legal' vs 'illegal arguments) it would be very telling of the way that media consumption habits have trended over the past decade or so.
The record industry (as opposed to the music industry) was a 20th century anomaly. Prior to the invention of the gramophone, music was largely a participatory affair.. singing to pass the time while working, to celebrate at a social function, to amuse and entertain friends and family and to pass on folklore from generation to generation.
The record as a social object and the idolatry of the rock star were temporary aberations.
The live performance end of the music industry will likely survive, but once all the big stars of the last few decades die off, even that will likely shrink dramatically in size.