Copyright is described by its proponents as a "Monopoly held by the creator". Considering how much we take from the Public Domain, I'd argue that Copyright should only last as long as Patents do, up to a maximum of 20 years.

Anything beyond that is not personal property, but a literal monopoly which impedes creativity, seeing the many, many, many sequels which are terrible often made to keep a studio's grasp on an IP. It happens so often those are often called "ashcan copy" or "rights-retention" films, which wouldn't happen if Copyright law expired as naturally as Patents do.