Prohibition of alcohol brought the gang violence of the 20's and 30'. When prohibition ended, most of the funds financing the gangs went away, and there was a corresponding drop in gang related crime...they lost their funds to attrect new members.
The new prohibition (drugs) has done exactly the same thing, and when it finally ends, you will see the gangs loose membership...for the same reason.. no easy money no attraction.
As long as the government does not get greedy and think they can tax MJ or Cocaine to the price people are paying today on the street, it will make a difference in gang violence. If the governments get too greedy, and just "compete", the moeny from smuggling will still be there, and so will the criminal's source of funds.
The "war on drugs" is just as stupid as the original prohibition was, and only enables the police state and the criminals. It does nothing positive for the safety and welfare of the people.