There's no evidence that the "war on drugs" prevents these people from getting the drugs that they want. I've known plenty of people who were into drugs. I've never known anyone who was into drugs and had a problem getting ahold of them. There are plenty of people who can't afford gas that can afford meth. You really think price is the issue?
There is, as mover aptly points out, a country that recently decriminalized all drugs, and after several years, the levels of drug usage have continually declined. And really, the decline is irrelevant. The 'war' creates huge criminal organizations, packs prisons, and costs a shitton of money. So long as you don't have reason to believe that usage levels will skyrocket, decriminalizing drug usage makes a ton of sense.