I don't believe undecided voters even exist. Like. No. Seriously.
This is the most polarized election in US history. we know both the candidates, we know how they did during their tenure as president. we've all got our opinion on it. The only question is, really, "how many voters are going to show up"- and more specifically, "which side loses the most voters?"
Honestly, there was never a positive upside for the debate. It's just an opportunity to give them more soundbites or images of biden looking... well, old. (and give trump a bunch of one-line zingers. his base doesn't care about the truth, just that he 'owns the libs')