And Rome was a great power, and so were the Mongols, and the Ottomans, and the Carthaginians.....
All empires fall, it's only a matter of time. America will fall - as usual, a combination of economic, social, and political factors will be involved. (Counting warfare, too.)
Might be a decade, might be a century, but I don't think it will last anywhere near a millennia. It's just historical - nations don't last that long, especially ones as culturally diverse as America.