No way Jose. The concept that Western Civilization will decline and will end as its enfeebled countries become too weak to resist invasion and subsequently become overrun by other cultures it's not realistic in our case.
The fact is that the American culture dominates other cultures and we can see that in the US. Despite the many cultures that exist here there is only one that predominates. It is clear that in one way or another it is the American culture, which dominates, or attempts to dominate the rest of the word. The result will be an Americanized world far away from extinction.
Remember that the cohesive elements of our culture - the American Creed as Huntington describes it - is utterly despised by those who shape the ideas that inform our society. That matters. It matters perhaps far more than most of us would like to admit even if we fully understood its implications.
My biggest concern is that the American culture that's been dominating the world isn't Traditional American Culture at all, but American Pop Culture: Mickey Mouse, Big Macs, Madonna, Hollywood films, etc.
Traditional American Culture is an entirely different thing: Liberty, property, Puritan work ethic, rule of law, rugged individualism, personal responsibility, etc. Not only do I not see us largely exporting these things, but I see us discarding them ourselves.
I'm only in my mid-30s, but I remember a different America in my youth. No one locked their doors, a man's word was his bond, schools actually taught and disciplined students, etc.
Our national character has changed. The character of our immigrants has changed. They don't respect us or their new homeland, and one reason I think they don't is because we don't stand for the same principals we used to.