Illegal downloads surely had an impact in the 2000s era, but I think by now the amount of freely streamable content is by far the largest factor.

Unlikely - streaming expanded the market for music.

I recall a report that showed that p2p networks acted primarily like a broadcast medium, which tuned to the most popular works.

The entire free content model has arisen because of tacit acknowledgement of that fact.

Youtube, hell everyone, applies those lessons today.

And, as I pointed out originally, there was about an 8 year gap between demise of album sales and the prominence of streaming.