People no longer need to steal to support their habits, therefore 'crimes to support habits' go away. Heroin and Cocaine dealers go out of business. Without the easy availability of these drugs to potential new users, the addiction rates drop. With lower addiction rates, demand for the drugs goes away. Columbian Cocaine farmers start planting Tobacco, Coffee or other legal drugs.
Worldwide supply dries up.
No More Drug Problem. I don't think I have to say any more. Whatever we have been doing for 20 years is not working. The War On Drugs is Another Viet Nam, 'We Ain't Gonna Win,' but as a people, we can prevail. But then again, I could be wrong.