I'm not sure how creditors distinguish between expansionary monetary policy and currency debasement, but I bet it's subjective and diplomatically sensitive. Even if nobody wants to call it default, though, a high inflation rate or recent inflation instability will at least limit a country's ability to issue local currency debt, exactly as if they formally repudiated their debts. That's a big problem when you depend on foreign creditors to cover half of your deficit
intragovernmental loans don't count
, or when you consider that a mere return to 1990s creditworthiness would mean paying $1.6 trillion interest per year. In 2018 dollars, obviously. It goes up with inflation.
Printing money could maybe work if the US intended to stay off debt for a long time, but I'm skeptical that the US government could afford to keep going without an inflow of foreign wealth. I don't think the US will ever be able to properly repay that debt, either. It's more like an eternal lien on the US working class at this point.
QE wasn't exactly printing money, it's more like a demand deposit, or a low-interest loan using treasury securities as collateral. But yeah, I get your point. The thing is, QE didn't just cause inflation, it also massively increased liquidity in the treasury securities markets. So it also made US government debt much less risky. Just issuing reserves without a bond swap won't do that.
The thing about fiat currency is, it's not worth anything by itself. It's only worth what you can buy with it. I've used the "pile of stuff" analogy earlier in this thread: take all of the stuff your country can make in a given year, put it all in a big pile, and then if you're holding 1% of the outstanding currency, you're allowed to take home 1% of the pile. That's more or less how fiat currencies work, it's a share in the total wealth of the country, redeemable at any time.
So why does a government collect a tax? It's because they want some of the pile. If the government requires everybody to give them 20% of all outstanding currency, the government can then spend that currency to take 20% of the pile of stuff. That leaves 80% of the pile for everybody else to divide amongst themselves.
What happens if the government simply prints 25% more money, and keeps the excess for itself? Then the government can spend that currency to buy 20% of the pile of stuff. That leaves 80% of the pile for everybody else to divide amongst themselves.
But what happens if the government just... like... steals 20% of the stuff? Sends in soldiers, takes 20% of the pile, and walks away? ...That leaves 80% of the pile for everybody else to divide amongst themselves.
In other words, it doesn't actually matter how the government finances itself. The result is the same, it's only who bears the burden that's different. And that's the real reason our governments collect income taxes instead of printing money, because, as you pointed out, it's the rich and the creditors who are affected by inflation, and it's the workers who are affected by income taxes. Thus, printing money is effectively a progressive tax, because the outcome is basically the same as an explicit progressive wealth tax would be.