The problem Robinson does not address is a simple one. About 21% of the population identifies as either "liberal" (16%) or "very liberal" (5%) according to Gallup. About 40% of the public identifies themselves as either "conservative" (31%) or "very conservative" (9%). This leaves a balance of Americans who self-identify as "moderate" (35%). Progressives can probably be categorized in the "very liberal" slot, and while they are undoubtedly upset with what they believe to be a capitulation to the center by the Democratic party, they are missing the point. After the last two years, Democrats in general are seen as liberal, and if you look at the poll again, that would point to a desperate problem; they can't get elected as a liberal party. 21% of the population is not going to get you there. Republicans have a similar problem if they drift too far to the right, but they begin with a 19% larger ideological base, and 10% plus one is a much lower hurdle to electoral success than 29% plus one.
Robinson may not like it and progressives may be confused and angry, but they are marginal to election, frankly, and the Democrats have to move back towards the center to stay viable as a party.