Sure, Slaughterhouse Five was Vonnegut's big financial breakthrough, but by that time he was a very well-known writer with several classics, including Player Piano and Harrison Bergeron, and a Guggenheim Fellow, and made a decent living from writing full time. Not glamorous for sure, but in line with most very good writerd.
Far from demonstrating the author's thesis that "failure can ripen into art", his story is the story of a man that had no notable failures in writingy, who consistently produced great work, and continued to do so until he made it big.