Your def not stupid for thinking that. Thinking illegality decreases usage is certainly the most intuitive feeling about it, but in reality it's- for whatever reason- been shown to be the opposite of that, time and time again. In a few studies concerning how law affects usage rates of illegal drugs, in almost every instance where hard drugs have been 'legalized' or decriminalized the usage rates drop. Not just a little bit but it's usually a significant amount.
It's important to understand that legality doesn't imply that it'll be as easy to get as alcohol or marijuana, that one should be able to just walk into a gas station and get some crack cocaine. they'd available only through a sort of "maintenance" treatments or rehabilitation programs, needing a script from a doc and with therapy and help being easy available for addicts. They will already be in the system and rehabilitation will be far easier for addicts to get once they inevitably get tired of being physically dependent on a substance, without worry of penalty of the law.
One instance in which your intuition proved right was in the more regulated market- During the 2000's it's was very easy for people in a few states to get oxycodone from a doc, especailly in Florida, which led to a thriving black market, where that oxycodone was being distributed across the US from people in those states. In recent years, regulations restricted how easy it was to get oxys and made it easier to catch doctor-shoppers. This lead to less oxycodone on the streets, although it wasn't a long term success addiction-rate wise, since many addicts just switched over to the cheaper and more readily available heroin. It kind of demonstrates how restricting drugs when they are in a heavily regulated but legal environment like that can have a real effect on the streets.
Keep in mind 90% of the serious consequences of drug addiction is the money and health problems. Legalizing this sort of treatment program negates a lot of both issues, depending on implementation. If we implemented a sort of maintenance program for seriously addictive drugs, specifically heroin, a lot of addicts when then be able to get their lives on track.