Some people seem to think that if drugs are legalized, the cartels will dissolve overnight and everyone involved in them will go get jobs at gas stations. As if criminality isn't both a living and way of life for these people. It may hurt their profits, but they'll still be selling drugs and might step up other operations like pimping and black market arms dealing to make up for the loss.
The idea that illegal immigration will dry up with the end of the drug trade is also a major brainlet take. The vast majority of illegal immigrants are economic migrants. Cartel crime is only one aspect of the problem, and doesn't address the economic issues caused by a massive glut of cheaper foreign laborers displacing the native labor market, or the social tension from massive cultural displacement as large numbers of foreign-born, non-English speakers with different customs, values, and traditions move into regions and form their own ethnic enclaves, leaving locals socially and culturally isolated in their own homeland.
Drug dealing is only a single issue of numerous ones caused by mass immigration from south of the border.