Objectively speaking, America's actual run as a nation was remarkably short and largely ineffectual during the final thirty years with the marxist culture wars raging internally. One could be a conservative and say the real glory days ran less than twenty years, centered around the time I was born.
If you look at all the things achieved by other world empires during their customary two centuries in power, it's actually far greater in scope than anything America achieved. The promise lay in what America might have done, not what it did. The start of the culture wars in the '60s stifled the nation into ennui within a decade and everything since then has just been treading water.
People like me warned America after WWII that the Bolshevists would destroy it from the inside. That's exactly what they did and they did it rather spectacularly. After forty years of media control the average American is a babbling manimal in man-pants, hardly even human, a gibbering drooling porn-watching illiterate moron. From such stock empires cannot be sustained.