He's the "best that they can put forward" because he's the President of the United States of America already.
He sets the narrative. He can make policy. He is on TV every day for free. He can take AF1 anywhere he wants and create political theater.
The phrase "a bird in the hand is worth two in the bush" is tailor-made for this situation.
I assume what you mean by the last sentence is "Biden should dump the black lady because maybe more misogynist, racist voters would pull the lever for him" but I don't think they would personally. I'm not her biggest fan, but . . .
Once again I fail to understand this debate. ANYONE is better than Trump. ANYONE.