I was just been told that Mexico is actually run by the cartels. They will not be too happy if the border is shut. That is their entire business. The cartels control Mexican soldiers and probably police.
Drug cartels need LEGITIMACY to spend their money, send their kids to schools, etc, etc. They set up legitimate biz companies with their illicit funds, buy land, homes, etc. & thru such, bribe govt officials to lobby on behalf of their interests. Election campaigns are not cheap, & thus legislators are beholden to them.
The drug cartels have billions hidden away, but they know they cannot afford to pay off everyone & elevate lives. They may employ staff or goons but they cant offer jobs to every citizens. They may indulge in charities, but those few dollars will not last even days. Thus they would have to make use the State to keep its citizens satisfied, thru honest trade & economics to create more jobs & social expenditures for their societies while running their illicit drug operations.
Now that Humankind had gone tough on those drug cartels, they will source for other avenues to ply their trade. It is no longer as easy as it once was to send their poison & unwanted citizens across the US border now, & at the same time, their legislators & minister would have to comply with honest trade conventions for revenue to alleviate their societal woes, which the drug cartels' funds, even with they are willing to give up which they would certainly not, would not be enough for societal needs.
However, such issues are problems that those drug supporting countries would have to deal with. Humanity will have to go tough & close their borders to protect its own societies & citizens.