Copyright is a monopoly. It just happens to be a monopoly defined by the Constitution, and protected by legislation bought and paid for by Disney. It's a monopoly that, if limited to a reasonable amount of time, enriches our culture. If that monopoly is permitted to go on too long, it harms our culture instead -- though I admit it's hard to argue that putting Mickey Mouse in the public domain would improve American culture.
While I don't mind being able to profit from sales of my books for the rest of my life, my creations drew upon the culture around me. It's only fair that my work should eventually join the culture as part of the public domain after twenty years or so.