You're mental model makes some strange and seemingly contradictory assumptions. It seems like your assuming that would be drug users are completely irrational when weighing the intrinsic consequences of drug-use like large ill-health effect, and great risk of addiction, but are very rational when weighing the legal costs of drugs use. Maybe you can help explain this? I would be surprised if the legal costs were even a third of the long term damage done by addiction, not to mention behavioral studies tell us its not the magnitude of the punishment but the likelihood. And judging from people I've met hard drugs are more likely to ruin lives through addiction* than the legal system.

Also when people argue for legal drugs the vast majority are arguing for end-users to have the ability to purchase drugs through a regulated system, not the ability to air drone a pound of heroin to your house at a moments notice. The two most important regulations in my mind should be age restriction(to stop children) and a waiting period(like 3 months to stop impulse buys).

*Knew 4-6 people over my life who had their lives ruined through hard drug use, and no-one who's life was ruined through the direct legal consequences of hard drug use.