I vote for the nutty butcher, Kosminski, after whose admission to Bedlam (or wherever) the murders ended. Also, the police knew who it was and immeidately after they got him locked up, they quit assigning overtime to the force, as if they were pre-scient in their knowledge that the murders were over.
Other likely suspects are Montague Druitt - since he fits the physical description and the murders stopped after he drowned himself (and McNaghten seemed to think he was the man) - and Francis Tumblety, who fled the country shortly after the last murder. Joseph Barnett is the classic "usual suspect" since he was actually living with one of the victims, but he did not leave town and was not confined. He WAS questioned intently by the police for several hours, which may have frightened him off (but I doubt it).
This is one of those questions that will never be answered - like Judge Crater or Ambrose Bierce.
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.