Actually, Cornwell doesn't think he did stop. She attributes plenty of later murders to him that fit the Ripper "profile."
Whether you're convinced by her arguments or not, you'll probably want to check out the book. I keep seeing arguments from people that she definitely addresses in the book. I know this because there was a "60 minutes" segment about her theory and research. It hit the major challenges to Sickert (including yours), and she promised much more detail in the book.