Certainly the biggest issue is that heroin costs money, and addicts will do anything for their dose. Making it legal--but not free--doesn't stop this. But there are other issues too (child neglect, homelessness, accosting people on the street) that even providing free drugs doesn't eliminate.
Alcohol prohibition might not have been worth it (in the sense that the curbs on freedom were too expensive) but it definitely worked in the sense of dramatically reducing alcohol deaths, diseases, and domestic disputes.
We should certainly consider making it legal--but not because zero heroin users was the goal.
That being said, I don't think anyone expects criminalization to yield zero use. I think the goal of criminalization is to make it more difficult to obtain the drugs and to dissuade people from starting drug use and/or continuing it.
So what exactly is criminalizing it accomplishing?
I agree that people shouldn’t use heroin. But obviously prohibition is not working.
Just to throw all the cards on the table, I'm by no means what anyone these days would describe as "libertarian". I don't think drugs should be legal for the sake of everything being legal, I think they should be legal because making them illegal doesn't accomplish the singular goal of stopping people from using them, and creates a lot of other deleterious effects.