Yeah, murder is illegal too, but still the number of murders is non-zero. Maybe we should just legalize it?
Sarcasm aside, if the goal is to reduce consumption, criminalization does work. Repression, though, does come with its own can of worm (an euphemism, yes). It's up to the citizenry and its representatives to decide if the trade-off is worth it.
At the cost of how many people's lives that get jailed for doing drugs/smoking and not hurting anyone?
Telling people what they cannot do in the privacy of their own homes is a massive invasion of pri... Ohh, well England is par for the course when it comes to that at this point.
How many people's lives have been ruined due to them getting caught with drugs? Your "net positive" just focuses on using force to make people not have self agency to what they do to their bodies.
You know what would really solve crime and drug abuse issues? If we just gave people lobotomies at the age of 16. They will all be nice and complacent and due what they're told by the state.
This law will attempt to ban cigarettes. Estimate how many people will buy them and smoke them illegally. The number will not be zero.