They are not going to replace Biden. The time to have done that was months ago when it wouldn’t look like a desperation move or a reaction to Ryan, whom they have been acting like was a great deal for their side.
Biden serves as the idiot for Obama to supposedly look brilliant, and to deter anyone from getting cute and wanting Obama gone.
Could be the scapegoat obimbo needs to have if he loses. Then he can claim he was loved but Joe pulled him down. Sooths the ego.