I'm not even sure what you're arguing.
That instead the majority voting bloc, a more diverse constituency, one that more closely resembles the Democratic coalition that wins elections, who have chosen a nominee... should be disregarded?
To humour young angry white guys that refuse to switch into GE mode.
Because they'll revolt or something.
No, I'm saying that they're more substantial than people here are saying they are. Those people just don't go away. Obviously, Hillary's platform should be what the DNC uses a basis moving forward, but certain things Bernie wants, like reforming the primary process, should be taken seriously. At least to make the process seem more fair and to make it easier to vote.
I'll be quick to admit this is my first election. Maybe young people get angry every election, and their target just changes every election. Seeing the state of income inequality in our country, I don't feel like these people will just forget all the rhetoric Bernie's been spewing for the past half year and that ignoring them will just make them angrier. Or maybe I'm accidentally believing there's an actual (small) political revolution at hand, when in reality this happens every four years. I don't know.
edit: I think the reason he won't endorse Clinton is because he thinks his "political revolution" has staying power. He doesn't want to undermine that, so instead of helping beat Trump by endorsing Clinton, he'll help beat him by opposing him directly. I don't think he'll make much of an impact in that regard once the media moves on tho.