Or they get into the "protection" and kidnapping racket even more than they are currently. Sure their size would probably decrease but the amount of violence would skyrocket. At least for several years. The basically control whole swathes of that. They wouldn't give that up without a fight.
Hard to disentangle effects from other stuff that was going on. Prohibition was repealed in 1933, in the depths of the Depression. It was also the year before FDR signed a comprehensive crime-fighting bill that allowed FBI agents to carry guns and make arrests.
By pure numbers, crime did hit a 20th-century high in 1933, right after prohibition was repealed, and they didn't pass those numbers until the 1970s-early 1990s (and only just barely then). But this was also the economic nadir of the century. And the mechanism for the decline in organized crime was that many crime bosses were killed or imprisoned, so it could've been the crime bill. So yes, it is what happened at the end of prohibition, but it's hard to draw firm conclusions between the two.
It wouldn't be nice for sure, but it would drastically slow down and change the magnitude. The thing is.. crime exists in most countries. What makes a big difference here is the amount of drug money coming into the cartels pockets, that type of money changes everything. We're talking of hundreds of billions of dollars a year, that's no pocket change. If money stops flowing in the cartels would have to switch to other types of operations, sadly still vicious and violent, but nothing like what they currently make.
