California has legalized marijuana for many years and there's still a strong cartel presence in marijuana trafficking. Cartels still find a way to profit from marijuana, other drugs, and immigration.
I find the notion that making fentanyl available at no cost being good for society by removing cartel involvement to be completely nonsensical. The end result is still the same. You still have a substance on the street that is fatal in incredibly small doses, and being used as fillers in party- and counterfeit drugs.
Legalizing fentanyl isn't going to alter that dynamic nor change the drug's lethality.
So no, the criminal aspects just don't disappear. They merely adapt.