Let's be real for a second here: if it was Joe Biden leading in the primary polls by 50+ points (both nationwide and in the early states) with his most likely opponents being Brian Schweitzer and Bernie Sanders, literally nobody would be talking about how he could possibly be upended in the primary. The only reason this is even a topic of discussion is because of the vendetta many on Atlas and in the pundit class have against Hillary. That and "muh 2008", which has already been thoroughly debunked by anyone who took 5 minutes to Wikipedia Hillary's position in 2006 relative to what it is now.
Nobody would be talking about it because it clearly would be able to happen. Biden is a goofball, he's screwed things up before, he can do it again.