The Sicilian Mafia has gone mostly “legit,” in the sense that they operate in legal industries, e.g. olive oil. They just use criminal means to extract hugely excessive rents from those industries.

They aren't competitive internationally, though. And can't be unless they're also going to go down to Morocco, fight the Army, invade Tunisia and Algeria, and then force the extraction of high rents.

The amount of violence the Sicilian Mafia inflicts, while staggering, is an order of magnitude or more off from the cartels.

Human trafficking and gun running are the cartel’s other significant sources of revenue. Then there are extortion and protection rackets. You could legalize absolutely everything and you would still have cartels. In Africa they are run by warlords and drug smuggling isn’t a big thing there — anywhere you have poor people and a weak government, you’ll have some form of “cartel.” The product is irrelevant.

You could legalize migration.

In any case, even if legalizing drugs still leaves you with some cartels, you would still have fewer and smaller.