Try opening a history book, son. Countries are defined by many things, one of which is type of government. You and the China-analogy fellow are apparently unaware that China and England have undergone dramatic government changes.
I've read more history books than you've had TV dinners, Pops. Countries that "don't last forever" are relics of the ancient world, when there was somewhere else to go when the empire falls. Planet is all full up now. Countries rise and fall in influence, which has happened to us in the last century (up) and is extremely likely to happen to us in the next century (down) if we don't watch ourselves. In extremis, they may change their form of government or be swallowed by a neighbor, which is not terribly likely to happen to us.
I'm sorry to dampen the dramatic hopes of people with an apocalyptic vision and a cellar full of MRE's, but the "cease to exist" option isn't on the table. In fact, it's arguable that global political movement is toward a more American approach (mostly because freedom works to the greatest good of the greatest number), and our biggest danger is letting some other nation out-American us.