Because it won't just happen over time what you are suggesting. You are suggesting a brisk and fast rate movement in the FFR to 10%+ with the additional insane spending idea of getting companies to produce as much goods as possible for the government to buy for the war effort. If am a US creditor, I say fuck this - I'll go and get something safer and park my cash in a government that will remain solvant for longer. The US government can do all the business it wants in dollars, it's also the largest importer of goods in the world, so they better have a currency people accept before going rogue.

So yes, the largest creditors would dump US bonds, and thus the dollar will go down with it. If you intend on spending, you won't be able to raise new debt for this spending. The old debt won't be your problem, the US government NEEDS new debt to operate, plain and simple. This is what happens in the EM world, and the US is not protected from this either. Sure thing 60% or more of global trades are done in USD, but as with Russia, we see how countries can adapt VERY fast at the new reality of being cut out from the USD.

Again, "it's such a cliche to predict its collapse" - your scenario IS bringing its collapse, so I am not even arguing today about the collapse of the US, because the US is not in your hypothetical situation thank goodness.