Harris had to drop out last time because Gabbard of all people slapped her across the room during the debate. Warren would win this horrible matchup.
You know that the incumbent president also didn't do well back in 2008 when he was running before being vice president?
But Biden was not looked upon as a high-tier nor a frontrunner candidate like Harris was, and he was running in the same field as Clinton and Obama. It's only his status as being VP is what landed him as the nominee. Even then he barely won the nominee, with Warren splitting the progressive vote.