Mathematically, there is no way that the US can ever pay back what it owes to China.

Well, that is of course wrong. Because what the US owes to China is in US Dollars.  And the US can print as many of them as it needs. 

So keep calm. There is no way that The US will not be able to give back to China the unlimited USD it can print. 

If China (or USA) will be able to buy with USD anything in the future, that’s a valid question that will be answered only by World War 3, as soon as (and if) the Digital Yuan will become the international method of exchange.

That’s exactly the reason why USA investing most of its resources on weapons and not in education, healthcare etc. to make sure that the debtor is always stronger and less vulnerable from it's creditor. 

As long as the world accept the USD as a valuable currency, US economy is secured, safe and keep thriving. So no worry. Hakuna Matata.