Not having a criminal mind, I could only guess, but the heart of man is evil. I'm sure they could think of something.
Since the cartels have expanded into kidnapping, other drugs, etc., explain how they can possibly replace 2/3 of the revenue they currently get from marijuana.
Expand their markets on things they already deal in (thus causing turf wars, more violence, etc). Come up with new ways to satisfy the dark heart of mankind other than drugs. (Plenty of young girls out there that could be forced into the sex trade.)
Suffice it to say, the criminal element isn't just going to give up 2/3 of its revenue. They will find ways to fill that gap, and it won't be in peaceful, lawful ways.
The cartels are killing each other by the 1000's, and you want us to believe they are passing up opportunities to increase their revenues by $billions? My original assessment stands - your argument makes no sense.