The best argument for this is the fact that many
products of
the largest corporations today were derived from previous
works that entered into the public domain. They took those
works, built upon them and created yet more value and
innovation (not unlike the patent system). The point is that
if this free flow of creativity is stopped by holding up
copyrights nearly indefinitely, creativity may suffer. Sure,
people can always go out and produce their own characters or
works of fiction or whatever. But the more corporations
maintain control, the more power they will have to claim that
other works are infringing on their copyrights.
Often it simply isn't worth it to build upon something unless you enjoy
exclusive rights. The restoration of old films is a good example. The
studios that hold copyrights to old pictures spend huge amounts of money
to restore the prints for issue in DVD format precisely because they
hold exclusive rights and can make back what they spend. In the case of
public domain films, this simply doesn't happen. As a result, you
cannot obtain a decent quality copy of most public domain films because
no one is willing to spend money on remastering when the public domain
distributors are selling the film for $6.99 or less a pop. If you want
an example, go on Amazon and read the DVD reviews for "Royal Wedding" a
1951 Fred Astaire/Jane Powell film that went public domain in 1979 when
MGM neglected to renew the copyright.