American history is chock full of near-misses, twists of fate, lucky breaks, and obscure politicians who almost became leader of the free world. The vice presidency has been vacant 18 different times, sometimes for years at a time.
Every one of those vacancies represented a constitutional crisis that wasn’t dealt with until the passage of the 25th Amendment in 1967. There was no way to fill a vice presidential vacancy, and it’s not clear whether Congress has the authority to call off-year presidential elections. With no president or vice-president, the very legitimacy of our government could have been put to the test.