The chart of books in print on Amazon, by decade, offers a compelling reason to allow copyright to expire.[1] There are more books available from 1830 than 1930; more books from 1900 than 2000. The cliff in that chart comes when the indefinite extension of copyright began in the early 20th century. It blew a massive hole in our cultural record.

You'll see the same story in film, and in software, though we have no pre-indefinite-copyright data to compare to. There's an era between roughly 1930 and 1980 where works that would have gone into the public domain, no longer are -- and because they are locked away, they are being forgotten or entirely lost. We enjoy the Eternal Sunshine of the Copyrighted Mind.

Though it's not a reason to allow copyright to expire, it's also worth noting that copyright expiration is required by the US Constitution. A copyright law that didn't include an expiration date would be unconstitutional. In practice, of course, a continually-increasing-yet-finite time is hard to distinguish from forever.

[1] http://www.theatlantic.com/technology/archive/2013/07/the-ho...

You have no doubt benefitted from the richness of our culture, yet are seemingly willing to prevent those who come after you from doing the same?