Both parties cover way more people than they should, because of the incentives of the system. Without another party successfully grabbing a huge chunk of the Democratic base, they aren't going to dissolve or anything else.
Despite what people claim, (that they won't vote for the other candidate in the general election), they likely will. The only ones I could see not doing so would be some Obama supporters who normally wouldn't vote anyway.
It is easy to perceive a candidates success in the primary system as a shift of the whole party to the left. Research shows that, while the candidates themselves are becoming further to one side, the actual electorate is becoming more centered.
I don't think much, if any, happens to the Dems if they lose.