Drugs inc and Nat Geo aren't good places to get your facts or numbers ;)
If you decriminalise possession of small amounts, it's still not legal and you can still go after the dealers, who are the actual problem. Most Junkies neither have the state of mind nor the financial means to move to another state, so decriminalisation will not lead to a massive influx in users but it will give law enforcement more time and resources to bust dealers.