"I don't get the idea you are actually suggesting growth and expansion of the cartels is only marginally related, if that, to the drug trade" What I am saying is that the growth and expansion of the cartels is primarily related to changes in the patterns of distribution for Columbian cocaine in the 90's when the Mexican cartels became wholesalers rather than strictly transporters and their profits soared. More recently the development of methamphetamine production capability by some Mexican cartels, notably the Sinaloa group and La Familia, has sparked splintering and infighting. The slaughter and atrocities are not being committed over smuggling routes for marijuana.
While cocaine, meth and other drugs play a role in the Mexican drug cartels profit picture, as much as 60% of their profits come from marijuana. This article in the Washington Post indicates that the biggest threat to the cartels is the growth of small marijuana entrepreneurs in California as legalization moves toward a reality in that state. And as California goes, so often goes the rest of the US.