Ok. So the gangsters find something else to sell. Then what?
They're more likely to kill each other fighting over the remaining drug business. They can't all just start selling other drugs because there is limited demand for other illegal drugs. When we legalize marijuana and allow it to be produced and distributed through legal channels we'll just take several billion dollars out of the illegal drug trade so there will be that much less money left for these people to fight over. They'll kill each other off fighting over what's left. Some will get out of the business. Fewer will get involved with it in the first place. In the end there will be fewer out there in organized crime.