> US has $40T federal debt (+state, +municipal, +household) and probably 100T+ debt worldwide which the future generations have to pay.

Future US citizens are clearly not going to repay the debt, simply because they can't feasibly pay that much back in real terms. They cannot achieve the impossible. The magnitude of the debt is far beyond what it makes sense for the US to make good on.

Future generations of the US won't be able to borrow on such favourable terms (ie, people will stop donating to the US cause) and all the capital investment that would have happened in the US to power their consumption happened in China instead - but the effects of the debt won't look like repaying anything.

You don't repay in real terms, you repay in nominal terms. That's priced in. The US isn't going to straight up default but it'll pay you back with inflated currency.