Mexico does not produce cocaine and they only produce a small amount of heroin. They do, however, buy cocaine from South America and smuggle it into this country and distribute it. The government estimates that Mexicans smuggle in and distributed about 90% of the cocaine consumed in this country and more than 80% of the meth and heroin. They already supply most all the demand for these drugs in this country, so they can't exactly just switch to selling these drugs if we take marijuana from them. There's not much upside left in the market for those drugs.
I guess you missed his point. They will BEGIN to produce those drugs once the market for pot had dried up.