> If they wrote a book 20 years ago and it didn't sell much it's not going to sell now either, no?

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.