That threat is already there and has been for much longer than Sept 11, 2001.
You're confusing yourself with logic and principle. We're talking about McCain here. He hasn't responded to the danger of porous borders because it hasn't been politically costly to him. As things ratchet up in the Middle East, the political cost-benefit equation will change, and may even change on the southern border. And the student-visa border.
He hasn't responded because he doesn't CARE what other people think.
When he tried the shamnesty he found out what the average American thought about it.
He already KNOWS that the average American wants our southern border secured.
He doesn't CARE what others think or want.
HE knows best what is right for America and that's all that matters to him.