Eh, no, the debt is not a problem, as long as the world is willing to pay the tax for the police and ordering institutions. The ruling empire is a defacto service provider- and the world taking and hoarding the dollar is basically allowing for the printing of endless debt, as long as the empire does its job. If it ceases to perform its job, the tax paid by buying and keeping currency reserves becomes worthless - and the debt realizes.

Thus can be concluded, the moment a empire acts only in self interest, abandoning all its services to the global public, the debt it holds becomes real.

The US will be able to keep issuing as much debt as they like as long as the Dollar is the global reserve currency. Its when that stops being the case they'll be in trouble with hyperinflation being the most likely outcome.