The assertions here would be true if we had a system that said people who lived in the United States when the United States ran up these crazy debts are still responsible for paying those debts even if they move away from the United States (or move their wealth out of the United States which is essentially the same thing). But we don't have that system and people are moving elsewhere.
This isn’t a hypothetical scenario. There was an article posted on HN today where Warren Buffett was essentially moving his wealth elsewhere (or "Betting against the dollar" as the article put it). So this is already happening.
Yes, right now the value of all the wealth U.S. residents have is way more than the debt. But as the Government tries to take more of it from wealthy private citizens and corporations those people will start finding ways to move that wealth elsewhere. Once wealthy private citizens and corporations start to move elsewhere the middle class will start to lose their jobs or get paid less. Which will cause the wealth that can’t be moved to diminish and leave the U.S. with a bunch of debt and citizens who can’t pay it.
The Credit Suisse analysis depends on the U.S. always being the richest and most desirable place to live but that’s not a guarantee.
On a side note it's also relevant that the Rich always find ways to subvert taxes. Look here: http://tinyurl.com/93jbf. While the point of the article is the debt pay attention to the revenue. You'll see Tax revenue remains remarkably stable. Even though there was a 90% tax rate at the beginning of the chart.