I think part of the idea is that whether the creators are dead or not, having copyright and "unrelated people feed off", still maintains an economy over the item and gives it monetary value. Whereas if it was de-copyrighted it would lose that value -- everybody could just copy it.
E.g. Disney only being able to create or allow the creation of Mickey Mouse stuff (t-shirts etc), leads to a large business for them, taxes for US etc. (Plus pays a lobby to ensure longer copyright). If Mickey Mouse was suddenly public domain, that would stop and its characters would be devalued very fast, with everybody competing with ever cheaper products with him on.