The idea that all copyright material is the result of a commission is invalid. Novels, for example, are frequently written before a publisher has been found. Depriving me of my income by abolishing copyright is not the same as industries that are no longer relevant. More comparable would be to pass a law that theft is not a crime.
Say you have two cars and I steal one. You've still got a car; you can still do all the things that people who own a car do, so what's the problem? How about if I mug you, but only take half the money in your wallet. You've still got money, so no problem?
Just because the income I might derive from my creation is less tangible than your second car makes it no less real. I accept the possibility that novels may go out of fashion, so no-one buys my books. But that my creation is given away without my permission is just not fair. And, in the end, that's what the law is for - to see that everyone gets treated fairly.