What’s the most credible argument that the current president can’t run for re-election yet can continue with the world’s toughest job for another 100 days?

under this model, its more about biden's trajectory, ie he will be capable until election day but not capable of being president for another term, thus he can stay as president but can't run again

China invades Tawain on September 1st. Biden has to make the calls. Can’t remember the name of his secretary of defense.

These things you throw out as "happens" or "what ifs" are all just hypotheticals!