As much as I'd like the paper's conclusion to be valid, it appears to ignore the fact that creating salable creative products has become much easier and cheaper in recent years. Practically every computer sold today comes with an audio mixer and video editor.
All we can conclude is that weaker copyright protection has not offset the positive effects of technology on creative output.
In average only about 15% of an artist income seems to derive directly from recordings. If this is a general rule for well established artists they should all release their music absolutely for free and invest their energies in concerts instead.
This would imply that, rationally, rappers should be more concerned about copyright, and people like Billy Joel and Neil Diamond shouldn't care one bit.