People stop giving huge wads of money to violent criminals. Drug users stop being legal pariahs and are able to be a part of the criminal justice system other than as just a defendant. The courts stop being crammed full of drug users. Police departments stop being preoccupied by "dope on the table" drug busts and concentrate on things like homicide and violent crimes.

No, gangs aren't going to go away, but they will be reduced in capability and law enforcement will be far more effective. Crime will go down. Not instantly, but over a fairly short amount of time.