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.

But even if that's true - does printing more dollars affect the price of crude oil on the international markets for instance - if the interest accrued on those loans exceeds the loss of value due to inflation, then they're still ahead overall. And that's the sort of thing that people take into account already when making investments.

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".

This is all true, but look at the context of discussion... Why would someone be discouraged from making future loans if the government printed money for the purpose of paying them off? Because the government took action which directly and intentionally reduced the return on their investment beyond what they anticipated in their original analysis. That will effect their analysis for future loans, reducing the appeal of those loans. Thus discouraging them.