I'd question if there's even a decline. One has to remember, that when you choose your data, you can present any picture; whether it is an accurate, unbiased measure of what really is going on is a whole 'nuther story. FWIW, I've got a LOT of new (to me) music here, and I don't think ANY of it was purchased new last year, so all that probably isn't counted. :scratch2:
This one is based upon "RIAA end of year shipments" whatever the heck that is. I doubt it includes used sales, including eBay and other outlets, and I would consider those to be not trivial. And how about Indie and direct from artist sales??? I'm not even sure if includes downloads from places like Amazon. Without knowing what constitutes the measure, it's hard, perhaps even ludicrous, to draw any real conclusions.
Unless of course, you're selling an agenda and using select data to support your contentions....