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…
What if the problem is that Dems in general are blamed for "bad vibes" (i.e., post-COVID malaise)? Then swapping Biden for somebody else won't help, but you lose the advantage of incumbency.
