It's not about the cost. Carmack talked about it in one QuakeCon keynote. It's much, much simpler to render two views on one screen than trying to use two physical screens.

Could you please explain how this proves that file sharing is hurting them?

Firstly, you'd need to define exactly what you mean by "growth". Amount of revenue, profits, people employed? None of those imply that the growth should be equal to the number of CD sales. For example, the profits might well be increasing by 10%, while the CD sales increase by 5%. All this means is that the industry has been able to increase their profit margins. Not that piracy is hurting them.

In fact, the logical conclusion to draw from your data (if it is correct) would be that the music industry is finding alternative revenue streams apart from CD sales to drive its growth. Either that or they're cutting their costs or increasing the price.

To get a more accurate picture, you should look at the development of the CD sales over time, and preferably compare it with the development of the customer base. If you see that the sales growth is going down as file sharing increases, you might postulate that file sharing is hurting the industry. But of course that would not prove it, there could be other reasons for it, such as the fact that most of the "music" sold these days is utter crap.

-Sunny