Your argument for the solvency of the US gov't is that it can't fail because its failure would mean the failure of the entire world economic system and that has not failed since 1700. In a word, it's never happened before so it can't happen now.
No, that wasn't my argument. My argument is that if the system was upheld by random factors, it would have collapsed long ago. It stands for reasons, and as long as those reasons stand the system is upheld. I named what I believe to the the reasons.
I'd have to go back and examine the stats, but there have certainly been many periods where the world suffered deep recessions and depressions, particularly in the 19th century before gov'ts, central banks, and int'l organizations existed that understood the stabilizing role of centralized control in the economy. I can name periods like the Great Depression when world trade fell precipitously and economies suffered under deflation. We had a similar episode just a few years ago. So if we haven't suffered a complete collapse, we have moved from from a stable equilibrium, if not in fact come quite close.
The role of depressions is overexaggerated by those suffering them. Because the world economy waxes and wanes does not mean that the relative position of the U.S. is fundamentally damaged relative to the other economies.
Britain suffered in two ways in WWII. Its coffers were drained by the second of two major wars in a span of just three decades and further stressed by the need to rebuild its war-shattered infrastructure. And it lost most of its former colonies. At the same time, America came out of both wars not only unscathed but rejuvinated by wartime investments in plant, machinery, technology, and infrastructure. Essentially, the US was the only strong and healthy man standing. So the mantle of guarantor of the world's reserve currency naturally fell on Uncle Sam's shoulders.
Britain lost its colonies because Japan took Singapore and showed once for all that the hold Britain had over them was mainly psychological. The U.S. stands in a similar position to vast expanses of the Third World, except that the U.S. outsources a lot of the actual implementation to dictators and supportive regimes. The U.S. took the global mantle after Britain's collapse; it was the U.S. which played the balancing act by working to destroy the new continental power, U.S.S.R.
If the US finds itself in the position of the UK after, say, a future war, and Europe or China somehow manages to escape as the US did in WWII, then the responsibility will fall on its shoulders. The Euro is already accepted as an alternative reserve currency. And should the US suffer heavy military losses while Europe didn't, then the EU could step up its military spending to assume the role of a military power. Likewise, China is quickly positioning itself to challenge the US as both an economic and military power. It isn't there yet, but the comparison is to some extent a relative one. If the US declines vis a vis China, that may be enough to reposition the two powers in the global theater.
The whole point is that the US won't find itself in such a position. The U.S. is more powerful than Britain ever was relative to its rivals. The EU is finished as a military power; they've been a virtual protectorate since WWII. They were only saved from U.S.S.R. by the U.S., which did so because that's how this kind of power works; you create problems for the rival center of power that resists the orbit by surrounding it with as many enemies as possible.
Think about this objectively and ignore all the debt hype; is the U.S. more or less dominant after the fall of the U.S.S.R? Before there were at least some restraints on foreign intervention, whereas now there are none. Is there any other country that we know with near certainty can invade any of its neighbors with impunity?
China is overhyped. They are surrounded by U.S. created protectorates or former protectorates like Japan, Taiwan (right on the Chinese coast, preventing them from projecting their navy abroad like the U.S. does), South Korea and others. Obama's fabled pivot to Asia; it didn't happen, but what was that about? It's about the beginning of pressure on China.
In addition to this, if you look up the statistics on the amount of value added in Chinese factories, you'll find that it is very minimal. China is much hype and little substance; in practice they have been converted into a big factory for the assembly of cheap goods. It is far easier to do this than it is to develop research facilities and industries necessary for developing new and competitive military technology.
At any rate, I think you grossly underestimate the risk posed by the amount of debt in the US and around the world. The economy has been propped up by artificial and unsustainable measures. At some point, these policies will fail to work, and then there will be a reckoning. The dollar like all other reserve currencies before it will lose its status one day. And that day may be a lot closer than many would like to believe.
Sure, the economy of the U.S. is propped up by artificial measures. This has been true since WWII at least. How else do you maintain such extraordinary wealth disparities, both domestically and in comparison with the world? But this doesn't mean that such artificial measures can't continue and even increase. Someday the U.S. will lose its position, but I find it hard to imagine that it will be in the foreseeable future.
Just think of it this way. The Japanese, a U.S. ally, did great up to a certain point. But then they ran out of things to copy in the West because technology peaked out. And their lack of R&D came back to bite them. The same thing will happen with China. The developer is tremendously advanced with respect to the imitator.