An author of fiction may sometimes retain copyright for some forms of derived works, ex "movie rights" [0], with the expectation that they aren't worth much until the original literary work has drawn a significant audience.

[0] https://youtu.be/Rjv1NlMiUyI?t=65

If they have done any research into it or have a halfway decent agent, yes. A book that sells well is probably not going to set you up for life and at best will just cover the next year or two, only a tiny minority of writers get that massive best seller and movie deal that sets them up for life.