You and I share the same opinion — we really are just another empire that will fall as all empires do. One can’t help but look back at Kipling’s White Man’s Burden (yes, it was racist, but that was still acceptable at the time) to see the UK’s own struggle with the decline of empire. I call him the UK’s “sunset poet”, the herald of the Empire’s decline.
We’re falling more quickly than the others did, but I suspect that’s due more to the ever-faster-paced nature of modern civilization than to weaknesses inherent in the American system. As you say, something may be lost once we have fallen…but look again at the UK. Even though they are much weaker than they were when the sun never set on their empire, they are still quite relevant and respected around the world today. But then, that might be because they’ve forgotten more about diplomacy than we Americans ever learned.