I agree with this proposal however it will never happen. You've forgotten that our government is corrupt and how many government/state and federal workers would be laid off if they were no longer chasing down drug dealers/users. Vice squads would be a thing of the past and the DEA would no longer have much to do.
Most libertarians and even conservatives that are social liberals would agree with all of this and it's not a new idea.
They need to re-organize law enforement anyway. The DEA could be used to make sure that uncle sam is getting his cut from these now legal, businesses.
Vice can use their talents to focus on organized crime and gang actvities.
The extra manpower can be used to focus on violent crimes.
Rape, robbery, and murder.
We already lost the war on drugs. Time for us to make money off of it.