The US Government is able to borrow massive amounts of money at below target inflation, because it is absolutely trusted not to default. People are literally paying us to take their money. This is a great situation to be in.
If we do have a massive default, then no one will ever loan us money again. You may think we can raise rates to a point they will loan us money, but as rates go up, the chances of us paying back goes down, and so it becomes a death spiral. There is no entity large enough to bail us out, and because it would be a self caused error, there is no entity that would want to bail us out.
It would not just destroy the borrowing, but also the US Dollar. The US Dollar is based on the ability to repay Treasurys, which would be destroyed by this financial collapse. Make no mistake about it, it would not be a depression, but a dark ages. America would be done as a country, and collapsing as a culture.
Losing its biggest trade partner would also hurt China. But it would hit America hardest.
Actually, the US dollar is not based on the ability to repay the bonds. Its value has no connection with them whatsoever.
The value of the dollar simply comes from the willingness of the people to use it as a medium of exchange.
If people lose that faith, the dollar will collapse.
This is true of any money, whether it's paper, bits on a disk, gold, silver, clamshells, pretty rocks, whatever.
Almost all nations today use fiat money (paper money with no other commodity attached to it). There is one exception: Iran, which uses a gold backed currency.
Gold has a history of being used as money for a good reason: It is easily checked for purity, and it is easily standardized in units of weight. It also does not tarnish.
Silver also has a history of being used as money for good reason: It is easily minted into coins, and reasonably easy to check for purity, and is easily standardized in units of weight. It does tarnish, however. Silver's role has mainly been in use as small transaction money, since gold is too precious for that.
In the United States, gold and silver were used as money, specified in the Constitution. That has never been amended. Gold for storage of wealth and large transactions, silver for small transactions like buying groceries or other sundry supplies. FDR confiscated (stole) the nations gold (and later the silver). To store all that confiscated gold, Fort Knox was built, since the Treasury building was too small. Since then, the U.S. has frittered away it's gold on wasteful spending. It is broke now. It has no more gold in Fort Knox. All that gold now belongs to someone else. It was FDR that put the nation on paper money as a replacement for the gold. It was Nixon that finally disconnected the dollar completely from gold. Since then, gold has been increasing in value rather rapidly, as the Federal Reserve Note (the dollar) has fallen in value, along with the rest of the fiat currencies in every major economic nation.
In the EU and Japan, it has gotten so bad that interest rates are NEGATIVE. This effectively says, "Take our money! We'll PAY you to take it!". Needless to say, these economies are NOT doing well.
We are facing the very real possibility that fiat money worldwide will be rejected within our lifetime. This isn't just one or two nations, like what happened to Mexico and the Peso, or Brazil. It is many nations facing this crisis at once.
Will the people turn to a worldwide currency like the SDR? Will they turn back to gold and silver? Will it be something like Bitcoin? Already some people are searching for another currency. Others are worried (quite legitimately) about the U.S. habit of printing massive amounts of currency to solve one 'crisis' or another.
The faith in the dollar is being lost.