Uh... no. Criminal elements work primarily by profit incentive. Organized crime multiplied after Prohibition, because the value of alcohol skyrocketed due to lingering demand for it. When alcohol was banned, people didn't suddenly stop drinking it. The only reason these drug cartels sell what they do is because of the profit margins involved. If we legalized pot (for example), the value of it would fall dramatically, and the cartels would drop it as one of their markets. Now, admittedly, this works best when both America and Mexico legalize something simultaneously, but even if Mexico didn't legalize pot while we did, the value of pot would be considerably less in America than it would be there. So, under that scenario, the cartels would still sell it in Mexico, but they'd have very little incentive to sell it here.