I bet you mean “legalize” instead of “decriminalize.” People often use those words interchangeably, but they don't mean the same thing. Massachusetts just decriminalized marijuana. I think if you are caught with an ounce or less up there they'll take it from you and give you something like a $100 fine now, just a ticket and it doesn't go on your record. Decriminalization usually means to remove the threat of jail time and in most cases it also means you won't get a criminal record if you are caught. Several states have decriminalized marijuana. If you are talking about a system where it is legal to grow it and sell it and possess it then you are talking about legalization, even if you anticipate that production and sales would be well regulated. Legalization with large scale commercial production of marijuana and sales through licensed retail shops is what would really hurt Mexican cartels.
Ultimately I would make it a licensed commodity like alcohol. Then the government would be happy because they would make billions in taxes from it. And there would be no pot buyers for the Mexican cartels.
There is still meth and heroin and cocaine, but I would not want to see those legalized, even though that would be the end of the cartels. They are just too harmful.