No Democrat is going to win the presidency without winning the votes of minorities by large margins. Without minorities Democrats wouldn't even have a chance at winning several swing states (if you remove large diverse cities from purple states they will turn red).
The black, Latino, and young Obama '12 non-voters were more consequential than the "Obama -> Trump WWC voter" who supposedly would have voted for Bernie if he were on the ballot. Hopefully Senator Sanders can evolve in his rhetoric on how the Democratic Party should galvanize voters moving forward. I have been very turned off by some of his assertions. I have seen him turn the corner a bit on that, so thankfully someone has gotten in his ear.
If we're talking about pure numbers, this is incorrect. There were about 6 million Obama --> Trump voters. If they all voted for Hillary instead of Trump, that would be a net gain of 12 million in Hillary's margins. You can turn out twice as many Latinos (which is almost impossible b/c that would be 92% turnout) and you still wouldn't get the same net gain as simply keeping all of the Obama --> Trump voters.
Turning out black people is a slightly better strategy, but even a whopping 100% black turnout would not give the same margin as simply keeping all of the Obama --> Trump voters.
I'm not saying what strategy the democratic party should do. I am just saying this strategy of trying to turn out young people + miniorities (and good f'ing luck turning out young latinos...) without focusing on swing voters is OK in the Popular Vote, poor in the electoral college, and bad in midterms... and a complete disaster in the Senate, which is comprised of many rural white states. The democratic party relies on a ton of old white people (they get about 40% of the old white vote). Even Obama got a lot of old white people to vote for him in 2008. Not as much in 2012, but still got plenty of them to vote for him.
Granted, I am of the opinion that by Nov 2020, Trump will be so damaged that anyone even marginally better than Hillary would easily win. But the educated white + young + miniority coalition is not as effective as people hype it up to be... you need to add something else to it. Obama added midwestern whites to this coalition.