I think you're correct in the vacuous sense that money is fungible. Investors who purchased US treasures ("the debt") don't care whether they're getting taxpaye

I think you're correct in the vacuous sense that money is fungible. Investors who purchased US treasures ("the debt") don't care whether they're getting taxpayer dollars or dollars that on paper exist because we minted a trillion dollar coin. They got dollars either way, and what they care about is how valuable those dollars are ("inflation") But in this MMT world, tracking US debt is still valuable (think of it as a measure of how much money was printed, which correlates through some unknown economic function to inflation). Either way, all the bad things that happen with printing money can be reframed as "bad because inflation" if you want them to. E.g. we print infinite money to pay off debts denominated in foreign currencies, which blows out the exchange rate and results in hyperinflation; yep, that's an inflation problem, check!