"Democrats lost because turnout was low. And turnout was low because progressives were turned off by their choices..."
First, regarding "low" progressive turnout: progressives turned out for Clinton in the same very high numbers as for Obama. (And note, further, that there was no party member or electoral 'bump' for Greens after Sanders was out.)
However, that turnout was what Moore - one of a minority of analysts to predict a Trump win - called a "depressed Sanders vote" - a vote that did not campaign, volunteer, or bang on doors to bring every other progressive to the polls. It's not that it was "low" in numbers - but low in energy. By running a right liberal candidate and excluding, belittling and ridiculing the large progressive minority that votes Democrat, the 'big tent' party cut itself off from vital sources of support that went beyond simply voting.
Second, however, note that it was not simply a "depressed" progressive turnout; the minority vote was "depressed" too - and, in fact, numerically lower than for Obama.
Triumphally and proprietarily claiming the black vote, the dominant right liberal wing of the party failed to gauge just how soft that black support would be for a right liberal candidate that might inspire fear of the right...but inspired little positive faith or enthusiasm either...leading to some minorities simply staying home...