...increase in the number of addicts
And why exactly do you think there would be an increase in the number of addicts? How many people do you know that just waiting to get out and pick up a heroin habit, if only it was legal?
Turns out there's just no evidence addiction rates will increase. There's some evidence more people might try a drug, but for the most part people predisposed to drug addiction are not being stopped by prohibition.
You may not be aware, since US media isn't very interested in going against the government, but Portugal has already decriminalized the possession of personal use quantities of drugs. All drugs. Cocaine and heroin too. They have NOT seen the number of addicts increase. In fact, the number of addicts GREATLY decreased, which is exactly what you expect to happen when addicts are treated like people who may need help and not as criminals who deserve a life-ruining felony conviction. (if you google for more information, be careful to note the distinction between addicts and people who have tried it, a lot of media outlets only report that 'tried it ever' has increased, and not that 'addicted to it' has gone down)