Let's Dispel with this fiction that...

[ag]

He cares a lot more about the future of the Democratic party than Hillary. Her supporters are doing the best to shrink the party. Bernie brings a broader coalition, as evidenced by his superior showing in general election polls.

Not true at all. Any extremist coalition is narrower than the one led by moderates. Sanders could bring in. some people on the fringe, but, long term (assuming Republicans do not continue running trumps and cruzes) he would lose many more in the center. One should not base long-term strategy on a few early election polls driven by the extremist offering on the Republican side and the anti-front-runner slander Clinton has been for such a long time exposed to. In any case, if Sanders were the nominee the same slanderers currently attacking Clinton would attack him: but unlike her he would not have a generally attractive policy package to counteract the character assassination.

[ag]

I mean...he cares about promoting his progressive values and policies, and could care less about the party. I don't really think that's a bad thing. I think values and policies should define the party, not the other way around, because parties can be bought by special interests, and bought parties can corrupt and pervert the issues.

If your values are not shared by most, it is a road to permanent losses. US constitution forces a two party system: that means both parties have to be grand coalitions of disparate ideological factions. Sanders is incapable of leading such a coalition. If you want an ideologically homogenous party to matter, you need to change the Constitution: at the very least you would need the Congress to be elected by some sort of PR.