If Biden had sounded this strong and energetic during the June 27 debate, he would likely still be in the race. Obviously, the set speech format is easier than having to answer questions on the fly. In addition, elderly people have good days and bad days (as do we all!). Today was a good day for Biden. June 27, not so much. The problem is that people in their 80s usually have a much higher percentage of bad days than before.
He's far from totally decrepit, nor is he suffering from senile dementia. But age-related decline does affect him. I am no fan of Kamala Harris. But I still think it's a good thing Biden withdrew.
I will leave evaluation of his legacy for another time. There are, I think, both really good things and really bad ones. And we may not yet have enough historical perspective on his tenure, in any case.