He's already got the source for the drugs. He's already got the organization to distribute the drugs and he's already used to making money off drugs. I don't think he's going to just walk away. He'll make some adjustments and then kill or intimidate the people who get in his way.
The organization to distribute the drugs cost money which adds to his bottom line. So does growing it illegally, as does selling it illegally.
Who exactly would you kill or intimidate to make your illegal drugs cheaper than legal drugs?