The goal is not necessarily to minimize number of drug users, it is to prevent people dying from overdoses (or other secondary issues like diseases), either because they have to hide their drug use or because there is no access to "safe" drugs (the drug your expecting, consumed in a sanitary way). Also, decriminalization can remove organized crime, violence, and other criminal activity from the equation, and make it less complicated to address drug abuse. The idea that it's just about reducing "addicts" is flawed. Decriminalization doesn't mean society thinks drug use is good, it's just the least bad option.