If Hillary doesn’t run, some governor will jump in. This will be like ‘92 when Bill C came out of the field.
In ‘92, Clinton was the last man standing for the Democrats.
In 92 all of the heavy hitters stayed out because Bush looked unbeatable with his 90% approval rating. The dynamic changed quickly and the country became quickly dissatisfied. Clinton + Perot equals not enough to electoral votes and Bill snuck through with 43% of the vote.