Cartels wouldn't have a business selling something like marijuana post-legalization. Just about anybody can grow it almost anywhere so its natural market value is extremely low. I doubt there are a lot of gangs financing their criminal activity off of potato sales or whatever.

Extortion and kidnapping must be way less pleasant and much more difficult than dealing drugs. It's very hard to believe that they could scale up to be the same sort of revenue source.

As for the US and its supposed success, sure, it's better off than Honduras, but it still spends a fortune on law enforcement and has a massive prison population. I don't want to think of how many lives have been ruined over needless drug-related convictions. US demand and money are also what's fuelling a lot of the violence in Latin America.