This is a really stunning claim and one that needs a lot more defense than you're giving it. I can think of countless innovative musicians, film directors, and authors who benefit from selling back catalog that's 10-20+ years old. I'd be happy to furnish a list if such a thing is necessary, but as it is your statement seems absurd on the face of it.
maldusiecle: I can think of countless innovative musicians, film directors, and authors who benefit from selling back catalog that's 10-20+ years old
I'd love to see more support for both sides of this argument. My guess would be that olau and maldusiecle may not have much overlap in the list of artists that they consider innovative. I'm probably closer to olau's position: while there are artists who benefit from copyright, they tend to be the ones who have chosen to concentrate on the commercial potential of their work rather than innovation. And I'd guess that most of those who are "doing art for art's sake" would do as well or better under a much less restrictive copyright regime.
But the problem, just like with patents, is that there a many kinds of actors in our modern world, including large companies who couldn't really care less. A company like Disney would, I think, be more motivated by a 5 or 10 year copyright period, as it would require them to keep on producing and innovating.
If we're talking about great, innovative artists, I think few of them actually benefit much from the copyright system anyway.