She can continue to earn money by making a competitive product. A copyright is a short-term monopoly granted to ensure that authors can recoup expense before the rush comes in. It's not meant as anything more than that. Whatever she makes in the first five years since publication is all she "earns" simply by holding a copyright. The rest of her money must be earned by contributing further to the progress of science and useful arts (perhaps by writing a sequel, which would have its own copyright term...) and/or competing on the open market with everyone else.
If the public takes the time to integrate your story into its culture, if they read your book and pore over the characters and develop their own interpretations and feelings, they should be able to share these commercially within a reasonable timeframe. When you publish something, it's not your own anymore.
I think Joss Whedon had a cool quote from a reddit AMA a while back. It was something like "Art is not a pet that you keep forever, which follows you around loyally without criticism until the end of its days. Art is a child, and it grows up and moves out and talks back to you." The real quote is probably better, but that was the concept.