The "simple convincing" argument is the one that you buy, not necessarily the one that's simple or convincing, right?
There are arguments that prohibition works -- look to homicide rates, and gun homicide rates. However, this wraps back around to the gun/car analogy. It's not a good analogy based on the fact that you can use a car to kill someone -- as you say, that's an inconvincing argument. However, where the analogy applies is that both guns and cars are useful tools, but carry a social cost (lives lost). In each instance, I feel the tool is worth the social cost -- the utility outweighs the "collateral damage." So, there are fair comparrisons of guns and cars, just not the ones you often see. IMO.