I just read that Border Patrol agents discovered a tunnel beneath the US-Mexico border. I looked at a video and it looked more serious than any Hamas or Hezbollah tunnel I've ever seen.
I think people underestimate the threat from the cartels. Right now, they're about making money so they have no interest in starting a big mess in the U.S. It's bad for business. However, they're armed to the teeth and as they say, "the blade itself incites the act of violence." 
These hundreds of thousands of well-armed, well-trained, and bloodthirsty men who can be turned from gangsters into invaders literally in one evening.
Tomorrow some nutter may decide he wants to start a Marxist revolution in North America, or free the occupied Mexican lands, or restore the Aztec empire or whatever, and then these guys will be able to cause terrible damage to the United States.
A weak border isn't just a demographic threat. It's also a military threat.