Imagine if all the classic works-- Dracula, Romeo and Juliet, Sherlock Holmes, The Red Badge of Courage etc.-- were all still under copyright protection. How hard would it be to craft a tale of horror, or a romance, or a mystery novel, or a tale of heroism? Pretty difficult without infringing on the ideas of these works. Some of the best work have been derived from what comes before it, and longer copyrights limit the potential of various industries. Sure, authors deserve to make money off of what they create, but not for decades and decades after they're dead and gone.
While I would be the first to argue that there is plenty of value in the copyright system n the United States I think that people need to realize that some things can be changed without harming people. There is nothing wrong with tweaking the current system for more modern times.
Copyright terms should not be shortened, particularly for literary and other artistic works, because it invites people to devalue these contributions and use them unfairly. The rise of technology has prompted a surge in theft of media (illegal downloading and sharing, in particular), resulting in profit loss (and employment trouble) for several industries. I believe this trend will continue but shortening copyright terms will certainly not stymie it.
I am the kind of person who believes that unless there is a very compelling reason afoot, something should be left well enough alone. The U.S. Copyright system is one such thing that I think should not be messed with. It is doing its job just fine as is, and should stay the same.