This debate has the potential to hurt or help Biden. Biden needs a win. People need to be energized about his campaign, because he needs high voter turn out to win the election. A loss for Biden, a neutral performance, or no one watching, in this debate all functionally do the same thing. Any of those cases is a missed opportunity. Biden skating by in this debate isn't going to cut it.
Trump succeeds when other people fail, so Biden's performance will matter more than whatever Trump does. Trump has a cult. Since there won't be a live audience, he could put a cap in Jake Tapper and Dana Bash and his supporters would cheer. So this debate isn't really for the MAGA crowd. It's for the rest of us.
I'm going to be incredibly frustrated if the headlines for the coverage after the debate is 'no one watched', because that can only help Trump.
Trump keeps dodging questions by going back to the previous topic. It gives him as much time to talk as he wants. And then he doesn't have to spend as much time on the issues where he is weaker.