As I reflect on the thoughts that the reading of these comments brought forward, I am worried it might diverge too far from the intention of thread. Nonetheless, for the sake of engagement:
By my understanding, copyright as a concept was instituted with the intent to encourage creativity. By protecting the ability of the creator to profit off his work, there was a greater motivation for someone to create. However, this protection was not intended to be infinite; after a period of time, copyright would expire, and others would be free to use the IP as they liked. The justification for this goes back to the nature of human creativity, in that it is always derivative; no idea is ever ex nihilo. Since human creativity is always inspired by something experienced or learned, no one can claim exhaustive ownership over their creation. Consequently, as the creator takes inspiration from the creations around him, he should not expect to restrict the inspiration others take from his creation.
This is far from a complete analysis, however; it does not consider the larger topic of motivation in creation (i.e., why does a person create), only its attempted stimulation.