If a copyright should preserve some, but not all, of the value of a musical piece, then we need to shorten that length a lot.
Winterspeak explains this neatly. The results in the post entitled "The Copyright Tax"show that the deadweight loss associated with our current copyright system (70 years) is only slightly less than the loss associated with an infinite copyright length. So, the 70 year limit is doing no favors for consumers of content.
This illustrates one of the great benefits of economics: being able to place unusual objective values on what are otherwise thought of as subjective concepts.