To add to this excellent post by @eschatonblog.com, the shitpost left also wasn't in a position to assess Biden's possible health issues either: it's not like we get many opportunities to meet with Biden (or advise him, for that matter). It was the supposed grownup centrists who let this fester.
I am really pissed off at the supposed “adults” in the room. Honestly, even if Biden were senile, it’s too late to change horses. Their job is to circle the wagons, defeat Trump, and then deal with whatever. They know perfectly well that it’s an administration, not a president, that matters.
just to add to this excellent point, most very successful people (in professional terms) haven't faced much true adversity in their professional careers (that's part of being very successful), so when things go off the rails, they panic. E.g., the NYT editorial board.