Old farts die. Plain and simple.
Billions of recordings have been made, and most sold over the hundred years of recorded music.
Old farts are the dudes with the BIG music collections.
They wind up either in a rummage sale, some then bought by scavengers who just want to resell them.. others by 'not quite dead' old farts who just gotta have that seventh copy of "Sons of the Pioneers" for luck..
Slowly they seep back into the system. And only a few wind up here or at eBay.
Eventually ALL the music collecting old farts will be dead. Except one, who has fifty billion recordings.
And when he dies... you will see even MORE recordings for sale.. but no-one to buy.
Everyone not-old fart will be using i-Pods.