They are already doing other things. Do you think the cartels are not trying to make as much as they can from human trafficking, cocaine and heroin, kidnappings, etc? What upside is left in any activity to replace the $8.6B or 2/3 of their current revenue?
There is no upside. That is why it is silly that some argue that the violence, et al associated with drug cartels would in any way be decreased if marijuana were legalized.