Can someone speculate on how this will actually play out in the coming decades?

US government bond yields are regarded as the rate of return on a risk-free asset. So, for someone to decide to lend you money or invest in your business, you need to offer them a higher rate of return. This extra return is the risk premium. So when interest rates are close to 0 per cent, mortgages are cheap and everyone is prepared to risk their money on a slide deck promising significant losses over the next three years. Because cash yields next to nothing, investors race to generate returns elsewhere, which artificially depresses the risk premium.

If US bonds are at 5 per cent, your banker will add their margin on top and your bank loan will be at least 7 per cent or higher if they fear a rise in interest rates and inflation. Investors will be much more discerning and demand higher risk premiums to move away from risk-free yields. As there are far fewer sectors capable of offering such returns, investment will concentrate on a very small number of sectors and companies (does AI ring a bell?).

It is therefore the bond yield that affects us directly, rather than the volume of debt alone. The volume of debt does have an effect, however, as the bulk of the interest is paid by issuing new bonds. If the government repays with cheaper bonds, it isn’t too serious; but if rates rise, the impact on budget deficits is exponential.

The thing is, the more debt and interest there is to pay, the more bonds need to be sold. To absorb this huge supply, the market demands higher yields to attract buyers, which in turn drives up borrowing costs for everyone.

International buyers will cool on the idea of holding US debt.

US policy will artificially lower the interest rate to stimulate the economy. Also, the spending of the federal government will continue to increase above the tax receipts.

This will result in inflation. Which is great if you owe money, since you are paying it back with cheaper money.

But, the interest rates being low will make it easier to borrow? The resulting divide by zero in the economy (meaning an exception that there is no solution to) will result in the US government repudiating it's prior debt and making the 'new dollar'.

This will cause a couple of years of chaos.

Anyone can speculate. If we fail to pay interest in a timely manner, then presumably fewer foreign entities will buy our debt, and we will have less money from debt sales to fund what debt funds. Whether that pain is spread equally or not is anyone's guess.

Scenario 1: It will be nothingburger right until the time it isn't. US can't default on its debt - so massive inflation and money printing until everything is rebalanced.

Scenario 2: AI and robotics deliver growth on par with semiconductors and internet. Couple of decades of the economy growing faster than the debt will put it in manageable ratio.

USA is really rich in resources, extremely well defended by geography, so I guess they will make do even in the worst scenarios.