Yes, criminal revenue comes in large part from drug trafficking, but it's not just marijuana. Local marijuana use in Canada is one of the problems, and marijuana exports as well. Canada has become a marijuana exporter country, but it is a major importer of heroin, cocaine and other chemicals.
Personally, I've always thought that legislation would have little impact on the crime world. The only effect would be to prevent a group of minor users from having a criminal record, which would be a good thing. As for the impact on organized crime, I don't think it would be major. The problem of drug trafficking is that they have a lot of cash—which I briefly talked about earlier—and they reinvest it partly in loan sharking, in illegal or legal businesses with nominee owners, and so on. It's this entire financial system that is the problem.