Maybe so but when in our history has a person not on the ballot been made president? Gerald Ford comes to mind but that is the only situation I can think of in which that happened.
None in modern memory, that's for sure. The primary reason I post is to dispel the notion that the party can't swap candidates after the convention. They can. Obviously they need to have a plausible reason. A substitute in this election would actually be legal, unlike substituting widow Carnahan.