> an artist should be able to enjoy the fruits of his work in his lifetime if he wants to

Not if you're an illustrator doing work for hire. It's not unreasonable or unusual for the company who commissioned the art to own the copyright. It doesn't always work that way, but there's no reason to think Kastel was robbed without us knowing the actual terms of his contract with Universal. I assume he sold the copyright to Universal, and Universal fumbled the copyright after that, but that doesn't mean it reverts back to Kastel.