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.