The worst outcome for Democrats in the long term and best outcome for Republicans is that he does so well that it increases his odds of staying in the race.
Biden’s interview with George Stephanopolous has taken on enormous consequence. It shouldn’t, really — Biden clearly isn’t up for this even if he aces the Stephanopoulos test. But the worst outcome for Democrats in the short term would be an okay interview — not something that immediately knocks him out of the race, but not something that is highly reassuring, either, so the “Is he too old?” discussion keeps going.
The worst outcome for Democrats in the long term and best outcome for Republicans is that he does so well that it increases his odds of staying in the race.