You're talking about public debt, which is a small fraction of all US wealth holdings (household + corporate + government), and can be trivially managed. Further, unlike a lot of countries, the US has significant spare taxing capability (it rests in the middle tier globally on taxation). The US could easily raise $200 billion in new annual income taxes on the top 25% and not miss a beat - that single move alone would handle any potential issues from the public debt cost. Or better yet: the US can constrain its spending growth (as it has been), allowing income and assets to continue to outrun the problem.
And of course the US owns the global reserve currency, which enables it to export inflation to the rest of the world as a means to reduce the domestic hit of things like QE (which is why the Eurozone's QE hasn't been as effective). When the US uses the dollar to improve its fiscal circumstances, the rest of the world foots a big part of the bill. While that's sort of like cheating, it's an advantage the US possesses that nobody else does.
You picked out the one thing that is the easy trump card: the global reserve currency makes it all that much easier for the US.
[1] - http://fortune.com/2015/10/14/1-percent-global-wealth-credit...
[2] - https://en.wikipedia.org/wiki/List_of_countries_by_distribut...
"Some 39 percent of the world’s wealth belongs to Americans, while Western Europe accounts for another 31 percent."
And that was before the huge asset rebound in the US, and was a drop of 12% due to the great recession.
http://economix.blogs.nytimes.com/2010/09/14/americas-domina...
Allianz's 2015 report indicates North America holds 45% of all global financial assets. Unless you're pretending that Canada and Mexico hold 20% of all global assets, your figures are wildly off the mark (Canada is less rich than the US per capita on average, and Mexico is far less so); at a minimum the US holds 40% to 42% of all global wealth.
https://www.allianz.com/v_1443702256000/media/economic_resea...
Yes, the US has debt, but if the market believes it's stronger than any of the alternatives... the US dollar still holds value.