You don't need to issue bonds. When government spends it automatically borrows the same amount at that time as a function of the way bank transfers work at the accounting level. Tax payments then reverse that as the money spins around the economy, which reduces that borrowing. What is left is simultaneously what people have saved from that spending flow, and the government deficit.

"You'd think if the government could get the same loans with the same terms without paying interest, then they would avoid paying interest."

That rather depends who government is working for doesn't it. If it is exempting holders of debt assets from the inflation process, by giving them free money when there is no need - while spinning an elaborate story to cover - it makes perfect sense.

"The more you increase the outward flow the more you need to increase the inward flow, and the extent to which you can do the latter is not limitless."

When you turn on the hose pipe, and then increase the flow by turning the tap, do you need to widen or lengthen the pipe, or does the water just come out of the end faster?

Sovereign money is a closed system. The more government spends, the more tax and financial savings it creates. Tax + financial savings = spending. Always. And it can do that until the cows come home.

What stops the process is running out of real things to buy at a price worth paying. Then government spending stops.

Government spending works like spinning a stone across a pond. Each hop is a tax point reducing the size of the next hop, and eventually the stone disappears.

Saving is like videoing the hopping and pressing the pause button. The hopping will then continue when the pause button releases - ie when the savings get spent.

"Nobody's explanation of this ever makes sense to me,"

Perhaps if you drew out the balance sheets, and applied the journals you'd see how it works.

What you're missing, it would seem, is that for every debtor there is a creditor and everything must always add up to zero at all times.

Do that and you'll see that what you're calling 'unsustainable debt' is just a balancing entry in the accounts.