Let me put it this way, the candidate everyone needs to vote for because defeating xyz is top priority always loses, if the candidate can not win organically in a primary they will lose in a general election.
Sanders is one of three that could very well defeat trump, the others are tulsi who has nearly no chance at all of winning the primary and amy who stood no chance at all until recently but has jumped into the viable category quickly. Sanders has major support and loyal supporters, and amy is a wildcard not enough about her support is known at this time to make any determination, So I would not rule her out.
But if it came down to it, bloomberg would likely face low turnout, he is a lifelong republican who backed bush and is a racist and his only major breaking from republican establishment is on gun control, while bernie could crank out the numbers, his voters are very loyal and active, they are much like trumps that no matter what the polls say they will show up in force to back their guy, and come election time you need to have you base plus independants, and more importantly you need them to SHOW UP TO VOTE AS IT DOES NOT MEAN SQUAT HOW MANY PEOPLE SAY THEY WILL VOTE IF NONE OF THEM SHOW>
Basically to do the democrat primaries justice, quit voting for who you think would win and vote for who you support, if you can only get support for the purpose of defeating xyz instead of because people back your ideas, you are doomed as a candidate.