This is assuming the only income is tax revenue (it's not). To put it very simply, federal debt is completely different than a credit card. You can take this point further.

Most of the US national debt is Treasury Bills. They are effectively bonds issued by the US government. You give the government $1000, and the government gives you an "IOU" that they will give you that amount back, with interest, at a later date. Think about that $25 bond your grandmother gave you 15 years ago that you caused for $35 last week. Usually they're short term, any time from 4 weeks up to a year. Interest rates are exceedingly low. you invest $1000 for a year, you might get $1005 back.

Those bonds can be traded on the open market. you can buy and sell them (where you are buying and selling the right to be paid money by the US government on a certain date in the future).

Virtually all of the "National Debt" is people who own these IOU's, and at some point in the future, the government will owe them some amount of money.

Of the $18 Trillion national debt, approximately 68% is domestic.

16% is held by the Social Security Administration. 13% is held by other US government entities 12% is held by the federal reserve bank.

So, 41% of the US national debt is held either by the US government itself or The Federal Reserve Bank, the De-facto central bank of the US. The remaining 25% in the "domestic" bin is owned by US citizens and US companies as investments.

34% of the national debt is "foreign" that is Treasury Bills that are held by someone outside of the United States.

China holds 7.7%% of the total - about 1.3-1.4 Trillion US dollars based on raw percentage calculation. Japan holds 7% of the total. Other countries fill out the remaining 19%.

These countries choose to hold these Treasury Bills because it is a ready reserve of US dollars. Those T-Bills are liquid and can be sold any time those countries need US currency. this is particularly important because most of the oil in the global marketplace is priced in US dollars, one of the reasons their value is so resillent.

Hypothetically, yes, China could decide one day that they no longer wish to hold any US dollars and sell them all. It's highly likely that dumping $1.4 trillion US dollars onto the market overnight would cause a disruption in the marketplace. It would cause the value of US dollars to fall a bit, but how much?

Well, look at this for comparison. Every single day$14 Trillion US Dollars flows through this country's economy. changing hands as investments and physical goods are bought and sold, debts are issued and repaid, people are paid, etc. $1.4 trillion is not, compared to the economy, a huge number. The price of T-Bills might fall, but there would be buyers for $1.4 trillion in US currency. The US wouldn't suddenly jump into hyperinflation because no one would want dollars.

Moreover, if china dumps their currency in such a way as it does hurt the value, they hit themselves in the pocketbook, taking a loss on money they invested.