No one has put forward the hypothesis that this might be caused by ever-lengthening copyright terms (till recently). I mean: surely they must have some effect.

No one has put forward the hypothesis that this might be caused by ever-lengthening copyright terms (till recently). I mean: surely they must have some effect. The old songs still have a (known) value, while new songs are still an unknown quantity. Since the copyright on the old songs lasts such a long time, there is less of an incentive to invest in new songs.