Even if Sanders is mathematically eliminated as a nominee, he can still push his agenda, like any other candidate. He still has a platform as a candidate, and a message to get out, and the fact that he rose from 3% to the high 30s and won several states says that his message has resonated. There are still many people who have no idea who he is or what he stands for, other than what other outlets have claimed him to be.
Plus,, like President Obama, he paves the way for those similar to him in the future. Obama broke the ceiling on African-Americans running for the White House. Sanders is doing much to normalize democratic socialism. Not that there isn't socialism already in this country, but he's making it acceptable to people who never realized how much of it we already have here.
Even if he doesn't win the nomination, that's okay. The best way to tweak the Republicans if Hillary wins, in my opinion, would be to nominate Sanders as her choice for the Supreme Court, if Obama's choice doesn't get approved. Think of the Republican heads a-sploding if she did that.