Any sort of carbon tax has no hope at all in the US! But pointing out that the optimal form is also the least politically acceptable is, IMHO, important for choosing how we address climate change and what sort of policies we choose.

Because "no hope" in politics can become "certain outcome" with only a few switches in influencers. The US is subject to minority veto in all of its national legislation, so if the Koch influence were to cease, or if there was a (shocking) change of heart from that huge influence on the minority political power that is the roadblock to a carbon tax, that could all change.