At this point, it's just hard to believe there are many people who are actually undecided. I'm sure there are lots who say they are. And it's really more about Trump than about Biden. Of the people who would ever consider voting for a Democrat, some love Biden, some aren't excited by him, a few hate him. There's a whole spectrum. With Trump, I think it's more binary, even among Republicans. They generally love him or loath him. I'm not sure I've heard anyone recently say "Hmm, that Trump guy says some things I like, but I'm not sure if he has my vote or not."
I guess what I'm saying is that I doubt a debate would change anyone's mind unless one of them said something to really alienate their base. "I want to see them discuss their differences on foreign policy before I decide." I don't think so. It's probably more risk than benefit for either of them.