別指望它還能支撐很久 - I doubt it. US is still in prime.

I will say it will take it at least another 200 years to become LESS relevant.  The key for its decline is not about FINANCE which is an ARTIFICAL factor, but when the SUPER MAJORITY of the US population are no longer White (more precislye Anglo-Saxxons). 

This is what happened to Roman Empire when the Romans were outnumbered by the Barbarians (who happened to be Anglos-Saxxons, mind you) and the new comers simply replaced Romans at every level of the society, INCLUDING THE MILITARY.  The barbarians eventually burned down Rome and established their own civilizations spread across Europe.