Until the US passed the Harrison tax act of 1914, we did exactly that. Opiates were perfectly legal to sell in over the counter medications, and there were many morphine addicts in the US. However, they could satisfy their addiction cheaply and legally, and so it was simply tolerated reasonably. They held jobs, they lived their lives, they just had to have their patent medicine every day.

The single biggest law that reduced addiction in the US was the 1906 pure food and drugs act. This required labeling the contents, and people started avoiding the ones with morphine.

In 1970, Britain did an experiment, they resisted the pressure from the United States and made heroin legal to addicts. Marginalized people who had been living on the streets, went back to their families, started working, because they didn't need to steal $100 a day to maintain their addiction. Then the US pressured Britain to stop the experiment, which they did, and the positive effects disappeared.

You made the glib statement above, but there is strong rationale to do exactly that. If we provided legal opiates, of a regulated strength, in a regulated manner to addicts, my belief is that we would see a dramatic drop in crime, and a serious reduction in the life problems of addicts.