If Hillary falls further then the Justice Department will indict her, so the Donk Party can put up the candidate of their choosing... But they have to wait until after the convention, otherwise they’ll be stuck with Bernie.
Between Judicial Watch and the FBI along with the increasing details of her racketeering, I don’t think Hillary will be out of the race before the convention. That means the Democrats are stuck with Sanders unless they pull a Lautenberg and substitute Biden. If they think Sanders’ supporters are unruly now, they haven’t seen anything.