The desire of the descendants of Jack Kirby and Siegel & Shuster to claw back rights to their antecedent's creations is in someways understandable (and in others looks like a pure money grab), but it highlights a number of oddities in the modern way of dealing with these types of story.
Whoever came up with Snow White maybe got a meal for their trouble, the attention of village for a half an hour and that was it. They got nothing thereafter from the legions who did very nicely from it over the ensuing centuries, including the zealous Mr Disney whose company has contributed so much to modern intellectual property law!
This way of doing business changed when the production of cultural artefacts became an industrial rather than a craft process. As soon as it became possible to easily manufacture creative properties (ie, around the time of the printing press, good old wikipedia tells us sometime around the 17th or 18th century) it became necessary to protect ones assets. Characters who might once have quickly passed into folk lore became something else entirelty, and as printing and distribution and literacy rates rose, the first pulp characters appeared - Sherlock Holmes, Fantomas, Carnacki the Ghost Detective (or whatever his name is). (NB- Like "pop", I think "pulp" is in many ways a description of the means of production and distribution rather than of a genre.)
Suddenly, exclusivity was important. The way these works were distributed was no longer a matter of tales around a fire, and there was more at stake for the creator than just a meal and bed for the night. But even then, the idea seemed to be that a creator should be granted the right to profit in their lifetime, but that after that (plus a seemly interval thereafter) there was no need to to protect these rights.
However, the humble business of telling tales became a huge industry and quickly things spun out of control, aided by the complicated nature of sheet music, films and music recordings. Fast forward, and here we are with children and grandchildren demanding the rights to things made before they were even born.
As a creator myself, I am of course concerned to monetise my own (modest) creative abilities as much as possible. After I'm dead though, well. I don't know. Do my kids really deserve to live high on the hog on Dad's toil? Are their hands any safer than anyone else's in regards to my legacy? Bearing in mind I'll be a defunct unit by then, should I even care?
More interestingly (or less interestingly, or just as tediously, depending on one's caffineated state) is the idea of competing franchises based on the same character. Comics fans famously have an obsession with continuity, but there's no reason why these characters should be treated any differently from any other folkloric character. They already have numerous competing franchises created by the current owners, aimed at different segments of the market (young children vs adults; casual vs committed readers; in-conitnuity vs out of continuity).
I'm genuinely torn in these cases, with sympathy for the families of Kirby and Siegel & Shuster, who were definitely shafted by today's standards but who did okay based on the situation that pertained at the time (less so Siegel & Shuster, IMO, who had their creation basically swiped; Stan had the good grace to rip Kirby off in a more legally defensible way). No one thought of Sipdey or Iron Man persevering that long, not even Stan and Jack, and who would have thought the rights they were signing away, adn seemed worthless at the time, might one day be worth millions and millions.
I guess that's at the heart of my ambivalence over this: are these people fired by a creative desire to serve the powerful and awesome (in the proper sense) creations or is it just a grab for the sudden seam riches that they've opened? The latter seems more likely to me. Old Jack wouldn't have spent his time in court, he'd have just sat down at the drawing board and created another mind bogglingly brilliant comics strip.