A case has been presented to limit the possibility of extending copyrights perpetually - the most balanced account of the case seems to bethis one.

At Lawrence Lessigs website an account if made of how copyright holders by not exercising their exclusive marketing rights are effectively removing copyrighted works from history - since no one can get at them.
It seems clear - with a few well-run franchises (Elvis and Mickey) as the exception - that this does not really hinder the cultural evolution where the arts are concerned. A evolutionist view of the story would be that since copyright holders are so ineffective at deriving value from their memes other memes take over and dominate our cultural space, rendering the discussion insignificant. A Case in point would be the rise of Manga comics in the west. It satisfies curiosities the old memes fail to adapt to. Further witnesses to the insignificance of the lawsuit would be the short shelf-life of contemporary music, the short shelf-life of most movies etc.

Of course I am a little too much of an old-world character to actually think like that. We nostalgics do believe in historical value. But still it is a persuasive argument even if you like old culture.

In the case of technology and science time limits are essential to the extent that ownership of old ideas induces ownership of the new, but the pace of invention itself means that there are time limits on the economic viability of an idea, since it will be rendered meaningless by future ideas not yet discovered/created.