The polling isn't all that close at the moment (if it was, there wouldn't be this call for him to step down). But I was talking about polling for his policies. They weren't always all that high. So the idea that a younger candidate can step in and simply say I'm running Biden's policies and voila, victory awaits, may be asking a lot.
IF Biden steps down his replacement will be Harris. Black voters aren't the largest in numbers, but they are the backbone of the Democratic Party. And we've already heard from some black commentators and Congressional Black Caucus members that IF Biden steps down you better pick the VP (instead of some other white candidate).
Harris could energize black voters who have drifted away from Biden and could possibly energize women voters. But she didn't even make it to the Iowa caucus for a reason. Being VP is one thing. Being the leader of the free world is another. She would have work to do to convince voters she's the one (definitely possible she could but she would be facing headwinds).