I want to make sure I'm understanding the advice here. Yes, we should want to write stories that are successful, but isn't the success largely based on finishing the product? Many more people will die wanting to be writers than will die actually having written. Also, to say that one person can independently determine whether a book would be successful feels a little narrow. J.K. Rowling is a prime example of someone who was passed up by a publisher before a different company saw the potential for her project. And these were supposedly experts. An author would make that person an expert on their own project, but I'd feel hesitant to place so much trust on asking an author to make an industry call that way. So if you were to tell this student, no, this would not sell, then what? Do they just not move forward with their idea? Maybe this is what you were saying, and I'm just too slow to take away that point.