Marijuana is just a small percent of drug sales in terms of dollars in the US. Even if you totally eleminated importing that, you would barely dent the illegal drug trade and the profits and business of the cartels. They will not go away if California and or Arizona legalizes marijuana. (granted these numbers are estimates and about ten years old).
http://www.pbs.org/wgbh/pages/frontline/shows/drugs/special/math.html
Chart does not copy well but in 1998, consumers spent an estimated $10.7 billion on marijuana and a combined $53.3 on other illegal drugs including cocaine, heroin, and meth. Cut them back on one and they will up production on the others. And what is to prevent them from getting involved on the legal side in the US as well? No- it is naive to think that legalizing marijuana will make problems go away.