Seriously? Giving monopolies to cartels drives massive profits. Cartels maintain actual military troops. It's hard to believe they'd have any source of crime remotely as lucrative without artificial monopolies making huge margins. Sure, they would do kidnapping, extortion, and so on, but that's not anywhere nearly the amount of easy money.
If the government granted Monsanto a license for weed, cocaine and poppy, the cartels would be cleared out in a month.
I fear they could coast on their cash reserves longer than that (and would desire to, instead of merely dissolve, to protect their remaining cash and avoid arrest), but otherwise I think you are exactly right. Kidnapping may be profitable, but it is nowhere near profitable enough to feed that beast.
If the government granted Monsanto a license for weed, cocaine and poppy, the cartels would be cleared out in a month.