By instituting a drinking age but if you are over that age, unfettered access?
Hence, 'pretty much'.
When I was in high school, illegal drugs were easier to get than beer. Hard liquor was impossible. My adult children say the same thing.
We also never hear about people going blind or dying from methanol laced hooch.
Illegal booze trafficking to children is not a thing.
Ending prohibition allowed for a safe, legal supply of alcoholic beverages to adults, ending the black market trafficking.
I don't think selling all currently illegal drugs OTC would be the best initial approach. I am thinking make a safe, legal supply of drugs available to diagnosed addicts, along with access to other treatments.