I'm not familiar with the laws in US, but from a commercial point of view, aren't the heirs trying to make the publisher pay for the author's bad business acumen? DC Comics made tonnes (& tonnes) of money from Superman, but when they paid the sum of $130 to Siegel and Shuster, did they know that it would become such a legend? It was a business risk that DC took, and like all business risks, that investment could have turned out to be a very bad one. With hindsight, Siegel and Shuster could (and should) have seen their lawyer, who would no doubt have negotiated a better deal for them. Perhaps they were desperate and financially in a very bad shape, so they accepted anything that came along, even (again, with hindsight), a "bad" deal.