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.

Watching his recent press conference and the NATO conference, it's clear he maintains a good grasp on things still, just has trouble elucidating at a moments notice. There's no reason to believe he couldn't make executive decisions, especially with the team he has around him.

Coinbase employee (Investigatooor), personal account. Onchain is the future.

His NAACP speech was more recent, more canned, more teleprompter and he couldn't remember anyone or even read through it. 
Probably the most cringe thing a POTUS has done in the modern era since Trump started his presidency by having his press secretary lambast the media with pics for saying the inauguration crowd was small.