We could very well apply this logic to say, murder. Why not let us legalise murder, set up appropriate channels for people to hire hitmen to kill each other, so that the underground killers would not have any more customers and then they will gradually die out?
Person A kills Person B, person B is affected.
We totally live in a deterministic world where 1 action affects only 1 person, and everything else is isolated from one another.
If you would like answers about the details you can search google or ask.
If you want to sound smart for figuring out that there are details, it's not going to happen.
What I'm trying to say is that when there are enough people using illegal drugs it is very difficult to use prisons and courts to get them to stop. Much like you can dam up a river but then the water flows around into other undesirable places. As with prohibition it seems a much more practical strategy to regulate the industry, if effect rerouting the river to where it does the least harm. Use regulation to make drugs safer and to minimize the harm to our communities when the the $100 billion dollars spent every year ends up in criminal hands.
like a government sponsored drone-strike-for-sale hotline? put the hitmen out of business with pure capitalism and good old legal (international) murder.