I think the activity of the cartels and it's influence on urban life in the US is as big a national security threat to the US as any middle-eastern country. There are little Mexican towns along the border with populations under 20K that have 1600 people murdered each year. The average soldier in Iraq doesn't face odds like that. I don't know what percentage of violent crime in the US is attributable to drug trafficking, but the number is unlikely to be low.
There have been numerous incursions into the US that are well coordinated, and use superior firepower, including fully automatic weapons that have leaked out of Mexican and South American military regimes. The cartels are well-known to have RPGs, grenades, SFRLs, etc.
I am not saying that the cartel activity has elevated to the point of military action, but it's not far-off, and entirely conceivable.
I am also not suggesting that select-fire is some kind of panacea to ward off ruthless gang lords, but it is certainly not difficult to imagine an escalation of drug violence that would create a vast disparity in firepower that our founders sought to prevent.
But, in such a scenario we have challenges to meet long before the question of select-fire in private hands must be answered, such as how to train a citizenry to be willing and capable of first responder duties until professional forces could arrive. This means local governments allowing trained, screened citizen to carry as a ubiquitous presence, for the purposes of their own defense, especially along the border States who are most terrorized by the criminals.
The reason the cartels operate with such impunity, is partly because of corrupt intermingling with the Mexican government at all levels, and because the Mexican people have no right to keep and bear arms. The chance of a cartel operative being shot by an armed citizen at home in Mexico is near zero. In the US, it might be 30%.
We are so blessed to have enjoyed relative safety on US soil for much of our history. But we should lapse neither into paranoia, nor a false sense of security. We are as vulnerable as we allow ourselves to be, no more and no less.
Tyranny, and terrorism, come in many forms. In Mexico, along our borders, and in our inner cities, it takes the form of Mexican drug cartels. To permanently take select-fire off the table as an option for citizens trained to assist police, in whatever limited role, is a huge mistake and it is antithetical to the original meaning and intent of the Second Amendment, which in part, sought to prevent the disparity of force which always allows a tyrant to prevail.