that argument maybe applies when you have a company such as Disney making a tangible economic benefit (to both the corporation and the economy at large) - although it's arguable whether breaking the monopoly gives more overall economic gain than what Disney loses.
but even assuming that it does, it only works when there's a huge empire providing thousands of jobs themed around a work, right? so how do you codify the difference between Mickey and $random_forgotten_book_from_1920, from which releasing copyright could only be a good thing for the economy?
I am beginning to think it should work like trademark, you own it as long as you are using it. Once you stop using it you have a short(in the case of trademark it is 3 years, but I could see 10-20 years being good for copyright) time before you lose it. That way it does what it should entice people to produce useful works, and let those works go free when they are no longer of use to the original author.
but even assuming that it does, it only works when there's a huge empire providing thousands of jobs themed around a work, right? so how do you codify the difference between Mickey and $random_forgotten_book_from_1920, from which releasing copyright could only be a good thing for the economy?