The legit channels have a level of convenience now not seen in the piracy channels, because money pays for proper curation and surfacing of content.

I used Limewire back in the day. I couldn't tell you how many songs in my collection were artist credit "System of a Down." I think I had a System of a Down copy of the 1812 Overture in there somewhere.

(It is also possible that legit channels are over-counted in the data because they're easier to explicitly audit. If the error factor estimate is the same for legit and bootleg channels, the data will reach the wrong conclusions).

That would be mostly due to the massive uptake of streaming music and mobile usage.

doesn't surprise me. most college students I've talked to recently (other than cs majors) genuinely have no idea how to pirate stuff (ie, don't even know what a torrent is). most are satisfied with the meagre selection on Spotify or even radio services like Pandora.