When they can no longer find enough people to willingly buy bonds at those artificially low rates, they will have to print money.
Oh wait, they already are - the Fed is now a huge “buyer” of Treasury debt, with money that they simply make themselves.
When that becomes prohibitive, they will have to force people to buy their debt.
Oh wait, they already do. They have increased the amount of Treasury debt that banks must hold, to be considered “solvent” by the Government regulators. Technically, they refer to that as financial repression (not a joke).
Then they will have to repay debt holders with IOUs. Already do, for a large percentage.
Then they will have to take more from citizens. Ditto.
Finally they will have to stop paying the bills. Party over.
A 20% import tariff would stop the bleeding and borrowing.