It would make sense to make it possible, but increasingly expensive to extend the copyright. It's fine for Disney to keep their successful creations, it just makes no sense to drag all other copyrighted works in existence along for the ride.
I'm not sure what your point is. Harry Potter is in the top ten highest grossing media franchises ever. Sure, top 10 franchises make a lot of money, but they're outliers. Is it in the public interest to grant every author in the country a copyright monopoly for life + 70 so that we can ensure JK Rowling's grandchildren continue to make billions from Harry Potter? Clearly not!