Once in a blue moon you people need to abandon your weird-ass views and think logically.
The ONLY basis for the worth placed in the USD is the faith in the US Government.
A default of the US Government undermines faith in the USG, therefore harming the USD.
That's not correct. A default makes the fiat currency stronger, not weaker. There's a difference between faith in the US Govt paying its debt obligations and faith in the US Govt-issued currency.
A default causes people to lose faith, yes, but they lose faith in the ability of the US govt being able to pay its debts. So when the govt asks for a loan, people ask for more interest. Bond prices go up because faith in the govt paying you back is diminished.
But bonds are different from fiat. When loans are less likely to be paid back, people want fiat. It becomes more valuable.
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You may not believe this argument, so let me put it another way. We all know the US govt is not going to be able to pay its promises. So it has two options:
1) Admit it can't pay its obligations. Cancel its promises. This is called a "default".
2) Print money to pay its obligations.
Clearly, the second option is inflationary. But what's less clear is that the former is deflationary. It increases the value of fiat money because people know the govt won't debase the currency. People lose faith in the ability of the govt to keep its promises and pay its obligations, but they know the govt will not debase the currency. Their faith in the currency increases.
Lol @ "cancel" promises. You can't "cancel" promises, you can only break them.
You seem to think that the value of something is based purely out of the amount of that thing that exist. It does, but only in part. There needs to be demand for that thing as well as limited supply, for that thing to have value. Since the USG effectively lied (that they would back it with their full faith and credit) to their bond investors, why would anybody have any reason to believe that they wouldn't lie to the owners of USD? Bonds are one of the things that determine the credit of the united states. Literally the USD is backed by the credit of the united states, it quite literally says so right on the bill. Just as a Gold ETF (note backed by gold) becomes worth less if what its backed by (gold) is worth less, so does the USD become worth less if the underlying (the credit of the united states) is worth less.
That's just the immediate effect. Should the USG default, there would be huge fallout that would likely make the dollar almost entirely worthless, at least outside of the United States. The FDIC is also 'backed' by the USG, but why would anybody trust that if the USG just defaulted on other debts? Why not withdraw your money from US banks which are now effectively uninsured and deposit into institutions ensured by the Swiss Government, denominated in the CHF not the USD, a government that hasn't broken its promises? That's just one other side effect, among many, causing USD outflows.
A government can't just default. It undermines the social contract between the government and the people. After all, imagine you're a bond investor, the government defaults, and then asks for taxes from you. You think you'd be OK with that? I agree that inflation isn't ideal, but a default would be orders of magnitude worse. A best case scenario would be uber high interest rates payable in assets the USG can't inflate; a worst case scenario would be anarchy.