To preface this, I am and have been conflicted on this topic as well. I lean a bit towards the possibility of finding a balance between efforts to reduce supply (e.g., some form of criminalization) while also treating demand (e.g., research into causes/factors leading to drug use, free substance abuse treatment, methadone/suboxone, etc.).

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.