One bad debate is one thing but this was maybe the worst debate performance in history. And Biden’s polling was already quite bad prior to this. These polls show essentially no advantage in sticking with Biden, which is highly unusual for an incumbent. It’s likely to only be worse after the new information gets factored in.
That said, you’re right that we likely need all of the information from new polling to decide on the correct path. But if democrats are smart they are at a minimum setting up an alternative candidate at this point.