The Value of the Public Domain by Lawrence Lessig:
Taxes are awful, but necessary. Let’s have them where necessary, but only when necessary. And so why not have them to extend the term of an existing copyright? BECAUSE THIS IS A TAX THAT CANNOT “INCREASE THE BOUNTY.” The work is already produced. No matter what we do today, Elvis is not going to produce any more recordings in 1957. So it is a tax that benefits some plainly (those who get almost twice the term they originally bargained for), but benefits society not at all. I.e., a very bad tax.
Exactly right. Lessig mentions an interesting article, The Value of the Public Domain by Rufus Pollock that goes into this area in more detail.