Consider how the Democrats fill vacancies on their tickets:

“Filling a Vacancy on the National Ticket: In the event of death, resignation or disability of a nominee of the Party for President or Vice President after the adjournment of the National Convention, the National Chairperson of the Democratic National Committee shall confer with the Democratic leadership of the United States Congress and the Democratic Governors Association and shall report to the Democratic National Committee, which is authorized to fill the vacancy or vacancies.”

So, to be clear, the vacancy would be decided by Tom Perez (a 2016 Clinton superdelegate), Steny Hoyer (a 2016 Clinton superdelegate), Jim Clyburn (a 2016 Clinton superdelegate), Chuck Schumer (a 2016 Clinton superdelegate), Dick Durbin (a 2016 Clinton superdelegate), and the DGA will decide on their own who will be our Democratic nominee.