I have canoed thru Big Bend. I can't imagine a more remote location in North America.
All of your points about the Mexican border are well taken. But my point is that:
1. We automatically assume any terrorist incursion will come over the Mexican border.
2. Yet, the Canadian border is every bit as vulnerable (and perhaps even more approachable) in this respect.
Thus, if it is terrorism we are really concerned about, we need to be equally concerned about both the southern AND northern borders.
If, instead, it is illegal immigration, then only one border is a problem.
Another difference is that at least in Washington State, when Americans who live along the border report aliens sneaking in, something IS done about it. This is not true of the Mexican border.