Harvard Business School calls bullsh*t: http://www.hbs.edu/research/pdf/09-132.pdf
As this essay has made clear, we do not yet have a full understanding of the mechanisms by which file sharing may have altered the incentives to produce entertainment. However, in the industry with the largest purported impact music consumer access to recordings has vastly improved since the advent of file haring. Since 2000, the number of recordings produced has more than doubled. In our view, this makes it difficult to argue that weaker copyright protection has had a negative impact on artists incentives to be creative.
There were a lot of other things that happened during that time that influenced the music industry. The paper is inconclusive.