As a libertarian, I can say that we don't often get to make emotional arguments, but here I think it's pretty clear that this is the only argument we need to make.
The thing Kleiman doesn't realize when we say drug use will go down from legalization is that while chronic addiction is a function of an individuals abuse and access to the drug, it is also a function of the ability of society to stop the user.
When you criminalize addiction, when you criminalize disease
, you stigmatize it. You stigmatize helping that person. Clean needle clinics in this country are treated like leprosy colonies.
Because of the War on Drugs we treat people who suffer from addiction to poisons as if the solution to their problem is to pile on more condemnation. Society acts as though there is some critical mass of condemning the behavior that will make a person stop engaging in it.
Drug addiction is solved and can only be solved through sympathy. When you criminalize a disease you throw up a massive social block in front of the natural instinct to help our fellow man. You're denying the most powerful tool we have as a species, our ability empathize, and replacing it with our worst: our ability to shun and hate. One of these leads to people getting treatment, and the other results in their being ostracized from society. It's easy to handwave it and go "just expand social support", but if you don't link up the legal code with our moral sentiments, you're going to face resistance.
This is why when decriminalization
has been adopted
relative drug use has gone down.