Are there that many people that would do drugs like heroin and meth but-for their illegality? Obviously, economics works at the margin--there are some people that get priced out of the market or view potential legal costs as too high--but I'd imagine demand is relatively inelastic; the people that would do it already do. Making it legal would at least allow us to focus our resources on those with real addiction (or at least trying to ensure they don't reuse needles, etc.). Drugs are a demand-side problem--people love to get high,
as Chris Rock once famously said--you won't solve it with supply-side solutions. Indeed, the illegality of drugs induces participants with high rents for those willing to take the risks of dealing.