Of course those on each side of a serious civic disagreement consider their own transgression of settled norms to be fully justified by the prior violation committed by the other side. That’s the logic of escalation. It plays out between nations in the run-up to a war, just as it can unfold within a single country that’s hurtling toward institutional paralysis, civil unrest, and an extra-constitutional power grab in the name of preserving law and order.
In the scenario Millhiser lays out, the left is fully justified in doing anything and everything necessary to enact a strongly progressive agenda because opposition to it is simply illegitimate. If we assume a political system based on simple majoritarianism at every level, and then also assume that a bare majority of the American people in 2018 would support a more leftist approach to policymaking than we’ve seen since at least the 1960s, then there might be considerable truth to this.
Yet note that even in this hypothetical case, tens of millions of Americans would presumably still oppose such an agenda. And not just oppose it — strongly oppose it.