A couple points and then a question.
There's a good argument that Democratic losses in the 90s and the early part of this decade were due to poor showings among non-college educated white voters. And as I've noted several times before, the idea that the constituencies you don't win in a Democratic primary are not winnable for you in the general, is literally nonsense.
But what about the general question of the 'problem' of Democrats consistently losing the white vote in national elections. Think about it this way. Since Democrats usually win upwards of 90% of the African-American vote in general elections, and African-Americans constitute something like 13% of the population, if Democrats consistently won the white vote they'd win every election by a crushing margin.
I'm sure Democrats wouldn't mind that. But it brings one point into sharp relief: though America's racial make-up is much more complex than just black and white, in the context of blacks and whites, the Democrats are the bi-racial coalition. They win elections by winning overwhelming margins of African-American votes and keeping the margin close among whites. (Obviously this is different in individual states with larger or smaller African-American populations.) Indeed, if Democrats continue to run strong, though not overwhelmly so, among Hispanics (something that seems probable in the short term with all the recent immigrant bashing on the right) this pattern could well become more pronounced.
There's nothing wrong with studying these percentages in terms of demography. Nor is there anything wrong with Democratic strategists recognizing that their candidates need to win this or that percentage of white voters to win. But creeping in the shadows of these conversations about how Democrats can no longer manage to win the white vote and are only saved from political oblivion by running up big margins among African-Americans is a little disguised assumption that African-American votes are somehow second-rate.
I don't think there's any getting around that.