The Romans didn't think the empire would fall either.
The Romans were on a bad position on the map. If they were isolated surrounded by seas like Japan back in that time, the kamikaze may have swept the Mongolia's fleets away. If the Romans were placed on America, probably they would have less chance being threaten by Mongolia. Too bad for them.
Some people then say that America should have several hundred years more of dominance, since we're not even three hundred years old yet.
But as a very astute historian once pointed out, it might be more accurate to measure longevity in terms of events, not simple chronological time.
Comparing our event-filled history to an age when it took several years after a declaration of war for opposing armies to meet, instead of a few days, one might say we've alreay lasted a heckuva long time.