On the other hand, if copyrights / patents lasted forever, the beneficial effects of competition might never kick in, and in the long run consumers would be the worse off for it.
So I see copyright / patents as a practical rather than a theoretical issue: what's the optimum term for the protection? What length of time will result in the greatest benefit for us all in the long run? Should it vary by type of creation maybe -- shorter terms for less capital-intensive creations, longer terms for others? It really is an optimization problem. The authors of the U.S. Constitution seemed cognizant of the tradeoffs and that they might need tweaking [2].
There was a fascinating story about a manufacturer of bra-rings in a National Geographic article about Chinese "Commodity Towns" a while ago [1]. I'm not sure what Chinese IP law is like, and what recourse if any the original inventor had, but it illustrates the problems quite well.
[1] page 2 and 3 of 8, http://ngm.nationalgeographic.com/ngm/0706/feature4/text2.ht...
[2] "To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries" http://en.wikisource.org/wiki/Constitution_of_the_United_Sta...