The only problem I see with a Trump v Biden debate is that Trump could be perceived as "picking on" a disabled person. Biden will become rattled and non-sensible. Trump will keep hitting him over, and over, and over. It could be perceived as piling on.
The flip side is that if Biden cannot stand up to the "evil Orange Man," how can Biden be expected to represent the U.S. on the world stage against our real adversaries?