Alcohol Prohibition took 12 years to go from "Great Idea!" to "Maybe we screwed up." It's been a century, and only because of Federal price supports have drugs staggered along.
Idea: Sell heroin and coke at true manufacturer's prices, per pound, 99% pure. That will kill all the old druggies, and if new ones are created, they'll die soon enough. It's not like they are an addition to the Social Order.
So you figure 5 million dead wood druggies die in your scenario?