The goal here is not to shrink the black market. It's harm reduction.
Plus, I don't believe that harm reduction without rehab efforts is good because it simply encourages addicts to keep using. I think it needs to be combined with another effort to help get addicts clean.
This is serious misunderstanding. Less than 50% of users are addicts. Usage of LSD, Magic shrooms and MDMA is usually for occasional entertainment, like at raves and festivals. People don't get addicted to shooms, and people dont generally consume 'party drugs' alone at home.