> This only works in pure anarchy where behavioral negative externalities are not paid for by others and people do not care whatsoever how much other people suffer, which is defensible, but not realistic.

I would argue the negative externalities are worse (especially factoring in negative externalities of a black market) under any approach that does not involve full legalization. Plus, legalization does not preclude taxation, which can be used for drug treatment and other programs that combat those negative externalities.