To steel man their argument, it's not about the polls now, it's about confidence in these candidates to fight into a lead. Harris doesn't look any worse than Biden here, and I think she'd have more ability to prosecute the case, so to speak.
That said…
The only way to make this situation worse is to give Trump the ability to say "I punched him so hard he had to quit the presidency in shame, and they had to send out their second string"
