The cartels have already branched out into other illegal activities¹, so the upside for revenue is limited. They are conglomerates of crime, and marijuana provides nearly 2/3 of their revenue according to the ONDCP.² Legalization would be a severe blow to the cartels.
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¹ They operate a range of illicit businesses from the regular extortion of street vendors to charging other groups for passage through their territory, to gun and drug smuggling, human smuggling, kidnapping for ransom, money laundering and the operation of a vast network of illegal businesses.
http://www.isn.ethz.ch/isn/Current-Affairs/Security-Watch/Detail/?id=97554&lng=en
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²John P. Walters, director of the Office of National Drug Control Policy, said marijuana, not heroin or cocaine, is the "bread and butter," "the center of gravity" for Mexican drug cartels that every year smuggle tons of it through the porous U.S.-Mexico border. Of the $13.8 billion that Americans contributed to Mexican drug traffickers in 2004-05, about 62 percent, or $8.6 billion, comes from marijuana consumption.
http://www.dallasnews.com/sharedcontent/dws/dn/latestnews/stories/022208dnintdrugs.3a98bb0.html
And then they would focus on their other avenues of funds. Cocaine, heroin, designer drugs, kidnapping, et al.