Earlier quoted context omitted.
If creditors are paid off, why would that discourage them from making future loans? Creditors surely want to get their money back, and countries that have trouble borrowing money are those that don't make debt repayments, not those that do.
Because printing more dollars reduces the value of each one, so they've been paid back with dollars worth less than the ones they loaned out.
And that's not considering that the same would happen with any alternative investments, nor the fact that risk is an important part of an investment calculation, and US debt is considered "risk-free".