He won't win. So what? There are higher purposes in running for public office than actually winning the election. That's less true when winning requires raising and spending a billion dollars, but if you can use your political capital to point out how utterly debased the system is and advocate populist solutions, go for it.
If the political tides change drastically and he does win, he'll be 75 when he takes office. So what? He's far more lucid than our last geriatric president.
He has voted for some icky pro-Israel resolutions. He also voted for the PATRIOT Act. These are not in the "so what?" category.
I am not anti-Israel, but I am anti–Israeli aggression, just as I oppose any government's aggression against its neighbors or internal ethnic groups. I especially abhor US aggression overseas and our police state at home. I wish more US Congressmembers, whether they claim Jewish ancestry or not, had the courage to stand up to Israel and AIPAC. I wish AIPAC and friends did not loudly equate misgivings about aid to Israel as anti-Semitism.
The massive USA PATRIOT Act of 2001 was written before 11 September 2001, but it was introduced as a response to it. Our legislators didn't read it. Their staffs likely didn't read it. They had only a vague idea of its implications. In the Senate, only Russ Feingold of Wisconsin voted against it, despite 66 representatives voting Nay in the House. What does this have to do with Bernie? It shows either a) how susceptible to peer pressure he can be or b) how he is willing to break from progressive orthodoxy and vote in ways that he perceives as beneficial to the nation. (Maybe there were lucrative contracts for Vermont firms buried in the PATRIOT Act; I'm certainly not going to read it, or even research online, and try to find them.)
It is important to remember, at least occasionally, that politics and governance are complex beasts; that some of the men and women we elect have complex reasons for their actions, whether or not we approve of those actions. Also, people change. Bernie may have once been a devoted Socialist, and may still talk a good game of redistributive economics, but the man has too much Washington in his blood.
The best reasonable hope for US Greens is that Senator Sanders will energize a sleeping electorate, lose the nomination, and convince those millions newly minted Progressives and Socialists to vote Green rather than settling for insert Democratic nominee here—I'm not falling for the Hillary Clinton Inevitability Narrative.