Probably little change. Drugs are readily available to anyone who wants them now. IMO, it is nonsense to argue that federal drug laws are keeping would-be addicts clean.
I don't think it is unreasonable to believe that addiction would actually decrease to some extent. There are big bucks to be had for drug dealers if they create addicts. That incentive would disappear if drugs were regulated in a manner similar to alcohol. Alcohol is probably the most widely abused drug in the US, but it does not fuel organized crime. There's a reason for that.
Now let me ask you. Do you think the Tenth Amendment should be honored and let the states regulate intrastate drug policies?
It's not the laws per se, but moreso the stigma.....Legalizing drugs somewhat removes the stigma.