The first stated purpose of copyright in the US is "To promote the Progress of Science and useful Arts". Keeping this in mind, first we can ask ourselves whethe

The first stated purpose of copyright in the US is "To promote the Progress of Science and useful Arts". Keeping this in mind, first we can ask ourselves whether the current system does or does not significantly limit progress by disallowing derivative works. If the purpose of copyright is still the same as it was when it was first specified, then our goal is to optimize for progress, and for that we need to ask ourselves at least one more question: to what extent does the duration of copyright prevent or stimulate potential creators to create. I don't think Disney had the income of his grandchildren in mind when he first started drawing. Perhaps the purpose of copyright has since shifted towards a principal of fairness and compensation for efforts. If this is the case I wish it wasn't: in my opinion the goal of society should be for society to progress on several levels, including culturally and technologically - at this level we shouldn't be interested in the wealth of individuals or corporations.