Democrats should study those people very very intensely and understand how they lost them. It was exceptionally radical to vote for Obama in 2008, people were calling him a cupboard muslim and terrorist sympathiser. They really believed he will deliver change and create a decisive break with neoliberal policy (both domestic and foreign), it is quite amazing that exactly these voters would vote 3 times for Trump after that.
Yet apart from Obamacare Obama delivered basically zero change in foreign or domestic policy. You simply can't take voters who went out of their way to vote for you for granted in this way and expect there won't be a backlash.
It sounds plausible, but I haven't seen anyone discuss it
What are you talking about? He got 68% of the electors; 53% of the population voted for him. That’s not radical: that’s mainstream.
The fact that Obama won so much of electorate implies that there were quite a lot of people who radically went against their usual political leaning. Those voters gave him the benefit of the doubt that he would shake the system.