The bottom line here is, the US owes-- as my economics professors (Reagan advisers) pointed out 25 years ago-- the one thing it can produce infinitely and with no cost: US dollars.
What happens in a meltdown? Well, if I ran the Fed along the principles my advisers taught me-- I'd pour payments to my allies (Europe, etc.); then I'd devalue the currency-- meaning, the creditors I didn't pay, now are owed much less. That's China, primarily, which, after all, has played quite a currency game with the US, and who's debt is quite arguably overvalued due to cheating on exchange rates.
Would China like the world to adopt the RMB as a default currency? Sure. Obvious. It's in their self-interest, at least. If everyone owes RMB, then they have to accept the global exchange rates.... they can't print or otherwise acquire USD, and pay off debts to China with the new currency.
Guess what? In short: no one's going to trust China as the global standard, as much as they aspire to it. If you're Switzerland, you know, that in a pinch, the US is going to pay off its debts to you-- not to China-- and give you a lot of notice, that it's going to devalue the dollar, so you can take advantage of the opportunity to erase 15% or 20% of your net debt to China, as well.
What's this called? Global political economics. China knows the game; it knows that it played the game to dump cheap labour and goods on the US, and accumulate capital debts (loans); and it knows, if the US reaches debt ceiling, it's China's debts that are going to get radically devalued to balance the sheets.
And good that. Unless, of course, you'd prefer that the Central Party, and not the US Treasury, is the final guarantor of the money you've loaned. A risk, I doubt, many of you (and many Central Banks) wish to take.
Or in short: the US will fulfill its obligations to allies and creditors who've played the game in good faith. To China and others who've gamed the system-- well, that's another story, and in the next days and weeks, we may very well see, what action the US can use, to re-balance the sheets.