Do you think cartels have an interest in drugs being illegal, and if so could you explain why? I’d expect cartels to welcome the change as they can do away with fighting the law (where they no doubt pour a lot of their resources into) and start operating as legitimate businesses.
Prohibition reduces competition which raises margins. It's the same reason why large corporations like regulation that destroys small competitors. The cartels don't want to have to compete with Joe Farmer in Iowa growing drug crops and selling them at the same margins as corn producers do.
Legitimate opiate producers compete in the same market with illegitimate ones. The force they use is police, dea, etc., instead of guns directly, but the techniques are the same.
After all, a state is what happens when organized crime expands its time horizons.
The state happened when people get fed up of being trodden on by tyrants, warlords and robber barons and decide to implement concepts like law, order and justice.
That it's not perfect doesn't mean it should be reduced to a trite, intellectually bankrupt, soundbite.