The gangs wont, but the addicts will stop committing crimes to support their habit, which will free up lots of police manhours, and just about all alternative forms of crime the core gangs would turn to are different from the drug trade in one key aspect - The victims will generally cooperate with the law when it comes to putting them behind bars. And without addicts doing stupid stuff and getting caught, lots of empty jail cells to throw the gangs in, too.