These millions currently on welfare generally are already drugged out dopes. Most inner city murders are committed to protect drug dealers territories and most burglary and robbery is to get the money for the drugs. You will have a huge reduction in crime with a minimal increase in consumption. Druggies are already using drugs. Availibilty is not a concern for these folks, but price is.
Your argument holds water only if there are no new users after legalizaion.
And do you really think THAT will happen? After Prohibition ended, millioms more started drinking. Same thing will happen with drugs.