A movie/tv show adaption of a book would be considered a derivative work. I could see that cutting into the sales of the source material, especially if it's bad

A movie/tv show adaption of a book would be considered a derivative work. I could see that cutting into the sales of the source material, especially if it's bad. Pokemon games are a great example of people buying it for the (copyrighted) characters, while the (not-copyrightable) gameplay constantly gets criticized. A game that copies the monster designs while improving on the gameplay would cut into sales. Other games inspired by Pokemon never have as good monster designs.