But I do like the idea of length determined by inverse correlation of size of the creator. 20 years might be too short where an author writes something popular and a movie company just waits 20 years to do something with it rather than pay the author.
That's not a universal rule. Andrzej Sapkowski wrote a little short story called "The Witcher" in the 80's, that he expanded on into a novel series through the 90's. Then a game development studio made a series of wildly successfully videogames based on his work, which definitely made way more money than his books, to the point that Netflix made a tv series based on his books. I struggle to imagine how it could be just that the videogames and tv show, based on his work, owe him nothing.
There's a reason why writers want their books to become videogames and or movies. I would not be surprised if the Tolkien estate made more money after the Peter Jackson movie came out than in all the decades before...
And most importantly artists are not children. If they don't have business sense enough to read a contract they should hire an agent.
Yeah, and why do you think he had those rights to sell? Copyright is a good thing, with flaws in its current implementation.