While it is fair to say that Clinton over all did better among non-white (primarily among black) voters, and Sanders had a lot of support from white voters, this has been exaggerated, overplayed and oversimplified to the point where I feel that it became, for some, a way of discrediting Sanders, of implicitly suggesting that his campaign and supporters were racist, that it was just the white people trying to keep minorities from being heard.
Sanders' support is heaviest among young voters, yes. This is probably the clearest demographic split of the primary. That's why it was specifically the racial element that I called a lie. While Sanders lost the black vote badly, there is some evidence that he did considerably better with other minority demographics, particular, again, young voters (as I recall, he roughly tied Clinton with young black voters, even, in at least some areas). Its just that with the exception of Latinos, no other minority racial group in America is nearly as big as the black vote.
Also, it should be noted that "young" is a relative term. Sanders wasn't just winning kids fresh out of high school who were eligible to vote for the first time. He was in the lead more or less up until you got to middle aged voters and older.
I also think its a mistake to lump the black and latino vote together, because unless I'm very much mistaken, Clinton pretty clearly did substantially better with the former than the latter. Though I admit I haven't looked at the relative numbers in every state, so I may be in error.
Actually, while many of the issues are very different, the result of the primary in this respect mirrors the Brexit results- the older generations shutting down the younger, more progressive generations in a relatively close race.
I'll also add that the Sanders disadvantage in primaries is exaggerated as well, in my opinion.
List of primaries Sanders won:
New Hampshire
Vermont
Oklahoma
Democrats Abroad (I'm particularly proud of this one, because I voted for Sanders in it

Michigan
Wisconsin
Rhode Island
Indiana
West Virginia
Oregon
Montana
Illinois, Missouri, Massachusetts, and Kentucky were also really, really close. I still maintain that Massachusetts would have gone to Bernie had Elizabeth Warren backed him before Super Tuesday.
So why does this matter? After all, as you said, he lost in the end.
Well, the issues and divides that drove this primary aren't going away. An accurate understanding of what happened now is important when it comes to what direction the Democratic Party takes in the future. Sanders lost. But he represented, at least, a very large minority of the Democratic Party and Left-leaning independents (maybe a majority when you factor in all the independents who couldn't vote in closed primaries?) which is likely to grow in the future since its support is predominantly among the (relatively) young. It is not, therefore, a group that the Democratic leadership can in any way ignore, any more than they could ignore the black vote or the latino vote or the womens' vote (not that those groups vote homogeneously either). I realize that I have some bias here, but I think that this is true regardless of what ones' personal preferences are.
As to your jabs about my taking too long to admit Sanders' defeat, I'll say what I believe I said pretty consistently from March-June- it was unlikely but possible for him to win. That was objectively true. I also feel that he was at least arguably justified in continuing his campaign for the Presidency up until California, and staying in until after DC as a matter of principle. While I would like him to get out now, I think its important that the transition to the general election happens in a way which will ensure the maximum number of his supporters come over to Clinton- which means giving them something besides just "not Trump" to vote for. "Not Trump" should be enough, but the fact is that for some people it isn't.
And really, is it so unreasonable? Clinton won the primary, sure. She should be the main voice shaping the direction of the Democratic Party for the time being. However, she and her loyalists should not be the only voice. Unity, especially in a large party, inherently entails compromise. I'm not saying that Clinton should give us everything. I'm not even saying she necessarily has to give us anything to win, though I doubt that it would hurt. I'm saying that to win by as much as possible, and leave the party in a strong place going forward, she needs to lead a Democratic Party that is responsive, welcoming, and respectful to all its constituent factions.
I listed four specific points I'd like to see earlier, which I will repeat here:
A Progressive VP, or at least one not part of the Wall Street insider crowd.
A position of some influence for Sanders in the DNC.
15 an hour.
Fracking ban.
Note that I picked the last two, in part, specifically because they would be relatively minor concessions from Clinton's own stances. Though it doesn't have to be those, of course.