When you say something like "Sanders can't win" then you've already lost. You gain nothing. You contribute nothing. You allow the ideas he stands for to be branded "unwinnable" and we're stuck exactly where we started. But if all that comes out of Sanders' campaign is that ideas like "healthcare is a human right" or "higher education should be free for everyone" become adopted by Clinton just to shut Sanders up, then Sanders has won. Look at Ron Paul. He never won the presidency but he gave a voice to those in the Republican party, and others, who sympathize with his Libertarian ideas. He won. The Republican party is more Libertarian because of him. That's what Sanders can do for those who consider themselves Social Democrats. But imagine Sanders actually did win. Then his ideas would really become powerful. And that can't happen if everyone who supports him throws up a white flag just as his campaign is getting started.