The debates have been very friendly towards Biden, to the point that nobody has actually challenged him to a serious discussion. It's going to take something incredible to keep Biden from falling into the following video now that it's only him and Bernie.
Go rewatch the debates, especially the last half, and see how Biden starts to lose his track.
If Biden does end up the Democratic nominee, I have no hope that he'll actually beat Trump in the swing states that Hillary performed poorly in by relying on his Obama-cred. Trump will absolutely destroy him in any debate and, unlike Bernie, has no qualms knifing where it hurts.