Not necessary, because the argument concerns “attaining” government power, which is not the same as “retaining” power. One is about getting power, the other is about keeping power.

The argument never compares democracies to nondemocracies.

Necessary, because if this were not true — if the advantage created by adaptability to public interests does NOT outweigh the harm associated with lack of sticking to principles — then we don’t have any reason to think that the policymakers’ lack of sticking to principles is more of a benefit than a harm. We’d have reason to think that it’s more harmful than beneficial.

Not necessary, because although the argument refers to policymakers who try to please different constituents to build majority coalitions, that doesn’t imply any specific requiremetnts about when they appeal principle and when they don’t. Maybe they can sometimes appeal to principle even if that principle doesn’t have majority support, because the minority that supports the principle cares more about that principle, and the people who don’t support it don’t care as much.

The argument never compares democracies to nondemocracies.