I see more value in having Disney pay an appropriately large amount of money to keep their IP than I see in making sure that their specific content gets into the public domain.
Who cares about steamboat willy if Disney is going to fight tooth and nail over it? It's everything else that will expire into the public domain that I care about.
It's not such a big deal applied to films; the amount in the government coffers is swollen by less than the extra Disney fans have voluntarily spent on old films, but no great loss is suffered by those who aren't willing or able to pay Disney's asking price. But establishing a principle that corporations should be able to retrospectively extend the fixed-term of their IP for cash would be particularly dangerous when it comes to things like patents...