Hillary is waiting in the wings. The short track for her is to be Biden’s running mate. Hillary offers reassurance to the establishment and will allow Democrats to overlook Biden’s mental failings and vote for that ticket. Hillary also brings her own supporters. If Biden looses his mind or health - Hillary is there to assume the Presidency.
Or if poor Joe were to realize his mind is going and commits suicide. I mean...I know it's a remote possibility, but the brakes could fail on that big old limo, or he could accidentally eat rat poison, or suffocate himself with his own pillow, or any number of horrible things.