What matters and what sells

Last month on Good Experience, Bit Literacy author Mark Hurst wrote about the sometimes-painful lessons he’d learned working with mainstream book publishers. “Drop any illusions about spending time with book lovers,” writes Hurst, “this is business.”
“Publishers and bookstores want a book that sells,” he warns. “[I]f your book will sell, it doesn't matter what you're writing about.” Publishers have one-track minds, but so does Hurst. Why doesn’t he flip that argument around? If what you’re writing about matters, it will sell, whether it’s a book or not.
If you read Hurst’s essay as I did, without having read his book or its title, you would have no clue what his book is about: A novel, a weight-loss regimen, a Western Civ. textbook, the results of his research on porphyrins? It doesn't matter what you write about, he seems to say, as long as it ends up being made of paper and ink and prominently displayed in a bookstore.
Why doesn’t this acclaimed Web innovator counsel aspiring authors to say, Here are my ideas. What channel(s) will they sell best in? Traditional book, e-book, web site, TV show, movie, etc.?
Don’t blame the book publisher for putting you through the old-school publishing wringer. That’s what book publishers do. But they’re not an author's only choice anymore.