Biden struggles just to read lines on a teleprompter, or to speak from notes. It doesn't seem likely he'd look good in a debate, regardless what they do for him. I seem to recall that he exceeded expectations in one of the recent debates, but can he keep pulling that off? He seems to be going downhill fast.
Has one of the major parties, in the mass media era, ever put forward a pres. candidate with Biden's level of impairment? I think we're in uncharted territory*, and I don't think we can predict what's going to happen. Other than Biden losing the general election, if he makes it that far.
*Yeah, I know we're already in uncharted territory, but this would be a different one.