Cutting the supply off without any alternatives absolutely caused harm. However, that only serves as evidence that safe and legal means of procurement are better than illegal ones.
If there had been a network of state dispensaries to provide safe regulated doses and support, over-prescription could have been stopped without leaving the victims to fend for themselves.
I'm not disagreeing about access to treatment, but just to be clear I don't see much intersectionality there with stopping the black market. For so many of them it becomes "sweet I can buy my black market shit, plus get whatever the addiction center place offers me on top of that."
That's the best outcome. In Canada the legal drugs they gave the junkies ended up being sold by mail to other parts of the country.
Not only are street drugs inherently unsafe, the dealers view their customers as so expendable they're willing to kill them for advertising.
To fix this we've got to switch to government supply, but only when we simultaneously wipe out the illegal supply and suppliers, so that we don't simply compound the problem. Our interventions are like antibiotics. They lose their power over time when misused. They're best employed heavily in a cocktail and used fully.
And the government has decided that drug users are on palliative care, that they're never expected to improve, so they can be given drugs that are likely to end or destroy their life without any concern for recovery. That sounds 'caring' but in areas without this assumption they actually do rescue many people from this death, making the 'caring' cities into uncaring hellholes for the addicted - where they go to die on the streets of a preventable death.
There is no fear of 'famine' because the people who are being given drugs are being given drugs for the rest of their life. There's no fear of running out because even if the government stopped the supply the street drugs are only a few dollars.
What these excess drugs do is 1) increase the amount of drugs the person is taking and/or 2) get sold outside of the drug cities, both of which increase the scope of the problem.
Also, druggies don't need money because they can just take whatever they want from stores and at most will be forced to give the item back. They're given food and shelter and drugs and can steal any luxury under $1000. (Not that they'd be punished above that, but the stores aren't required to sit and accept it - they can apply reasonable force to recover their goods.)
The way 7 year olds are kept out of rushing rivers, or consuming black market dope, is they have a parent, guardian, foster caretaker, someone looking after them. It's illegal to neglect a child. Sending a 7 year old to buy you cigarettes arguably isn't neglect, letting them smoke cigarettes or research chemicals probably is.
[0] https://en.wikipedia.org/wiki/Systembolaget