There is a third scenario which this article doesn’t address. The President has a natural understudy whose only job (other than to show up to the Senate a few times a year) is to take over in case a President can’t do his job anymore. So why are we all acting like it’s some big wildcard?
If any President had a major medical, condition – say, a stroke or heart attack – during a campaign it might be prudent for them to back out not just from the campaign but from the office itself. In that case, there is no question who takes over the ticket because it will be the new incumbent President.
Now, we all know Biden didn’t have any major medical episode. All he has is a sudden case of notgonnawinitis, complicated by inflammation of the opeds. But if he came out next week and said “I had a minor medical issue two days before the debate, my doctors said I would recover fully at the time but now they are not so sure, so I need to back out of all of it”, don’t you think the entire party would immediately (and enthusiastically!) hand the reins off to the vice-president, once they remember who she is?