Takes quite a stretch of the mind to entertain this hypothetical. Looking through the history of U.S politics, and in particular the more modern period, it seems very long odds that this would ever happen. Mainly because it would require all of it to happen at a very rapid pace. Pelosi or Harris would have their position filled within a few weeks. That being said, the fact we even have to think about how our line of succession goes through people who are all around eighty (besides VP Harris we have 82 Pelosi, 79 Biden, and 88 Feinstein/82 Leahy).....
We don't have age nor term limits why again? Most federal government agencies have a mandatory retirement age, but not the people in charge of them!