That’s pretty solid advice, I agree. Though I could not personally say anything about it. I think there you’d have to actually ask writers whose books aren’t selling. But whether they are willing to openly talk about it is another story.
Another way I think would work is to put a few excerpts up for reading and creating a poll to see how many readers would be compelled to read more about a certain story. But then this basically is the beta-reader thing which I’m sure many people are already aware of.
If I recall correctly, Andy Weir, author of “The Martian” published each chapter online for his readership for free as a hobby project and people became fans before he even signed a publishing deal. So it was direct reader feedback for him that formed the rest of the book, page by page.