Originally copyright was a bargain between the creator and the government (originally the Crown, but now the people) - essentially it's "we give you a fixed term monopoly, and in return you turn your creation over into the public domain afterwards".

Two things have messed with that deal - corporations are now treated as people, who live forever, and copyright extension allows corporations to extend copyright past the lifespan of the public they've made that bargain with.

I'd argue that a bargain that you can never collect on is not a valid bargain - the whole basis of copyright has been undermined - now it's just a monopoly.