"They assume that Mr. Mamdani’s unorthodox policy ideas and personal flaws are so self-evidently disqualifying that voters will reject him if they keep hearing how bad he is." What personal flaws? Practical politics will for unrealistic policy plans. Mamdani is aiming high, as he should.
I think what the Democratic establishment "assumes" is that they, not voters, are entitled to select the candidate, even when it is someone as corrupt, uncharismatic, and unimaginative as Mario Cuomo's nepo-baby. So yes, they are blindsided by genuinely exciting candidates like Mamdani.