-
I don't think in 4 years he's gone from ok-ish to a decrepit incoherent mess.
Some of us can testify to that being entirely possible in the older generation.
He's not in that bad a state, but that's not even the point. He failed under pressure in a critical situation, causing enough people to worry that he wouldn't be up to other critical situations. Add to that the people who don't care about the details of politics but want their elected leader to look strong and make them feel proud. I'd guess the latter group is larger.
Did you watch/listen to the debate clips?