For how long? They'll just forget about marijuana at that point and get serious about coke and heroin, and you can't legalize all of it.
According to our government, about 90% of the cocaine consumed in this country comes through Mexico, is supplied by these organizations. They're also supplying most all the meth and heroin consumed here. They are serious about these drugs. But there is only a limited demand for them. Americans consume more marijuana than all these other illegal drug combined. The black market for illegal drugs is mostly a black market for marijuana. These other drugs are expensive and profit margins are high, but most of the money is coming from marijuana. According to the ONDCP, about 62% of their gross proceeds from drugs bound for the U.S. comes from marijuana. They gross about %28 of gross proceeds comes from the next biggest seller, cocaine, but that must first be purchased and smuggled from South America before they bring it here. They don't produce it like they produce the marijuana. The drug czar, John Walters, says marijuana is the “bread and butter,” “the center of gravity” for Mexican drug cartels. Taking marijuana from them would be a devastating blow to these organizations.
http://www.dallasnews.com/sharedcontent/dws/dn/latestnews/stories/022208dnintdrugs.3a98bb0.html