Things are always sustainable till they aren't. The US can impose to USD as world reserve currency only with ever increasing costs.
The US is beholden to the Saudis and other Gulf states to sell oil only in USD. Saddam Hussein, for a long time America's best boy, signed his own death warrant by selling oil for Euros. That war cost the US $3trillion. Gaddhafi signed his death warrant by just talking about setting up a North African currency(gold backed). Iran is being sanctioned(quarantined) because it is selling oil in non USD currencies. Russia is being sanctioned for the same reason.
Without reserve status for its currency, the US would be finished, at least in its current form.
