You're ignoring a morally significant dimension to the problem. Nobody becomes a drug addict by accident. They choose to do drugs and take the risk not just of bodily harm, but of becoming unable to support their families and becoming a burden on society. And your suggestion is that society should spend money to treat them of an illness that was their fault to begin with.

I happen to agree with you that treatment, not prosecution is the answer for drug addicts. But there's legitimate moral reasons to oppose the idea.

Besides that, drug use is already well on its way to becoming decriminalized. The drug war is aimed not at users, but mainly at importers and purveyors. That poses another moral question: if drugs are harmful and addictive, why can't society ban people from selling them, like it does for all sorts of other harmful products? I understand the cost-benefit justification (bans are ineffective), but voters aren't driven by cost-benefit analysis. There are legitimate moral reasons to oppose the idea that we should just let people defy society and sell harmful products just because we can't effectively prevent them from doing so.