progress is promoted when people can use existing stuff. Just look at Pride and Prejudice and Zombies and other things remixing existing public domain stuff.

Sorry, I forgot how poorly sarcasm translates over text. I completely agree, and think that the current copyright period is far too long, preventing expansion on existing works.

I think Spivak's comment was talking about the "de facto" purpose as born out by experience. I usually don't like being cynical, but it seems copyright legislation is one of those areas where it's hard to be overly pessimistic