There are also drug addicts who would like to stop, if more money (that's currently spent on prevention) was being spent on treatment.

Read up on Portugal, they legalised all drugs, just over ten years ago, and the reports released a few months ago (after ten years of it) showed that there are less people using the drugs that previously were illegal (from weed up to heroine), and a higher percentage of those using drugs are in treatment to stop.