"Joe Shuster. Shuster and Siegel famously sold their creation to DC Comics for a mere $130 in 1938, long before the character went on to become a multi-billion dollar franchise "
I'm sorry, these guys created something then sold it. The company to which they sold it promoted it, worked on it, created new products that drew in new audiences and generally built the equity of the brand. Then, the heirs come in and demand a cut with the argument the original sellers were cheated because the product is now worth billions.
If Superman had been worth billions at the time they sold it, would Shuster and Siegel not have receive more than $130? It must suck to think of something, sell your idea for almost nothing and then see other people take it, invest in it, repackage it and then make a fortune.