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.
The undecided voters are probably the ones considering jumping from Biden to 3rd party over current hot buttons (e.g Palestine) than any ever would be Biden to Trump or vice versa.
Maybe if they're paying attention while Donald is talking about needing a "final solution" for the conflict they might realize he's not their man after all.
It seems Biden has started pivoting on Israel support. Slowly, and certainly not definitively, but he's been calling out Netanyahu when he hadn't before.
Depending on how fast this turn around happens a debate could probably swing a lot of 3rd party voters back to him. If he rips off the bandaid and admits that Netanyahu is in full ethnic cleanse mode.