Hmm, why do you think that the number of drug addicts will go down if drugs are legalized???
Current addicts were drawn into it not because it's criminalized, but despite of it. And they are using despite it been risky for their freedom and health and despite it costing a fortune. Addicts are using because it gives them a rush or a temporal "way out" of their problems
So what will happen when more safer (cleaner) and cheaper drugs will be available LEGALLY? I think more people will be willing to try them (those who were scared off by legal repercussions and other risks before).
What will bring the number of addicts down is drugs like "crocodile".

Current users will die fast, and those with an ounce of common sense will shy away from it eventually. The fact that it's very cheap and affordable is good too. Less theft and robbery all around.