@Lash,
I believe Carville & McCaskill are mostly motivated by real concerns that, despite his enthusiastic and loyal supporters , Sanders is very unlikely to be able to win a general election. However, I suspect we agree that they have themselves certainly failed to come up with a better or more electable alternative. Biden was knowingly a very weak candidate long before they orchestrated his resurrection, and I believe that Bloomberg will, for different reasons, also prove to be a weak or weakened candidate if he is selected by a then fractured Democrat party.
Sanders is a life-long socialist enthusiast who, I believe, ardently believes in the long term benefits of the socialist programs he recommends. I suspect that is why his rhetoric is so short on just how he would go about enacting (or paying for) them. One result is that he comes across with an authenticity and commitment that none of his current opponents (Sen. Warren, in particular) can match.
The problem, of course is that the history of the past century amply confirms that such programs are seriously enervating to the societies and economies that adopt them. This is widely understood by a majority of our voters, and, as a result, the likelihood of the election of a President advocating them, or, if elected, actually enacting them is negligibly small.