Didn't help in 2010 and 2014. What you are advocating for is a political suicide mission based on principle or the "true believer" route. Republicans certainly dont wanna go that route and neither do Democrats.
There is no way Bernie Sanders like others have eloquently stated could win a general. You cant just expect to nominate any candidate and win. Candidate quality matter a lot.
Congress matters but the Supreme Court matters much more this time. Republicans to be honest have a lock on the House at least till 2023 barring a wave next year or in 2018 or 2020. So that means that whatever progressive policies you may want from Sanders would be dead on arrival in Congress. You do not want a trifecta in 2017 overturning and appointing judges ensuring a conservative Supreme Court for decades to come.
In other words, we need someone who has the best chance to win and the Republicans can atleast be willing to work with and that person is Hillary.
"Republicans willing to work with" is rubbish so get that out of the way. (Republicans actually have been principled btw, just for principles that are horrible). They will not work with anyone and we shouldn't want to work with them, we should want to overthrow them by getting dems elected. If we don't get dems elected they are going to do what they want because they have a radical insurgency forcing the fire underneath their representatives (as the left should have). Republicans will be like this no matter who is the democratic nominee/president, if this elementary point hasn't gotten through to liberals after Obama's presidency then I don't know what else could.
As for this being about leftist principles, this is not an excuse liberals can use anymore. Sanders is not running as independent/3rd party. You guys can't throw Nader at us. Sanders is running within the establishment party, with all of the benefits included.
The democratic party is going to win this election because of demographics and the grassroots forces that have arisen in the last few years, for left-wing/center-left social and economic positions, and the level of support they get from various celebrities, media organizations, newspapers etc.
The only question is who is going to lead that party and advocate the kind of things liberals want (supposedly) and working people need. There is no reason not to fight as hard as we can to get Sanders nominated as he actually believes in the stuff we want implemented.
There is no reason to convince yourself the left wing position is so hopeless when this is a legitimate chance. If you thought Warren should run for president and had a chance at beating Hilary you should defend and support Bernie as he is her replacement.