Not sure I agree. Would Sanders, or Biden, be weaker than Hillary?
Biden is incompetent. He would purely be a figurehead, literally operated by handlers.
Sanders would actually be stronger than Hillary. He actually believes his message (she doesn’t believe hers), he actually wants to turn the USA into a Bolshevik paradise (she just wants coronation), and he actually has a strong following (she has an entourage). Bernie will really fight to win and to give away everything until the system collapses.
Trump v Sanders would actually make for an interesting knock-down drag-out contest.
Trump v Clinton would just be a hungry cat playing with a disabled mouse.