Now, there very well may be some people who create something and live off it for the rest of their lives without creating another thing. So what? If someone creates something so wonderful/critical/popular that it still generates income 50 years later why shouldn't they benefit from that?
Ok, so how does having copyright last until 75 years AFTER THE DEATH of the author benefit the author? They're worm food.
As far a Disney, they take from the public domain, but do everything in their power to prevent their works enriching that same public domain. In addition, after they make their work, they may attack anyone else approaching that same PD source work for another project. Plus, they don't always have the rights they think they do when creating a work (example: Winnie the Pooh by A. A. Milne. Disney stepped well beyond the license they had to the work.)