This idealised setup is no longer the case. The last US budget surplus was eighteen years ago in 2000. Since then the US economy has had a bust, followed by a boom, another bust and in 2017-18 was in its second boom. The accumulated budget deficits since 2000 total $12.454 trillion and government debt has increased from $5.674 trillion to $21.516 trillion. Whatever one thinks of Keynesian interventionism, the abuse by the US Government of Keynes's theory of the state's management of the economy is truly staggering, and seems set to drive it into a debt trap that can only result in either a severe retrenchment of state spending or the eventual destruction of the state's currency.
However, the rolling over of old debts and the continual addition of new ones will almost certainly become a problem for governments everywhere. It is less of a problem when the debt is put to productive use, but that is rarely, if ever, the case with government finances. To judge whether the rolling over of debt is sustainable and at what cost, we need to rely on other metrics. The traditional method is to compare outstanding debt with GDP, and by using this approach two economists (Carmen Reinhart and Ken Rogoff) came up with a rule of thumb, that once a government's debt to GDP ratio exceeded approximately 90%, economic growth becomes progressively impaired.
It turns out that the Reinhart-Rogoff report severely understated the problem by reporting early. Their 90% debt to GDP Rubicon has been left behind anyway, with government debt to GDP ratios around the world in excess of 100% becoming common. In the case of the US, total Federal debt, including intragovernmental holdings, is currently over 105% and rising. The Congressional Budget Office is forecasting substantial budget deficits out to 2028, adding an estimated further $4.776 trillion in deficits between fiscal 2019-23, or $9.446 trillion between fiscal 2019-28.
Our best-case outcome of controlled price inflation is essentially that forecast by the Congressional Budget Office. Working from the CBO's own figures, by 2023 we can estimate accumulated debt including intragovernmental holdings will be $26.3 trillion including our estimated interest cost totalling $1.3 trillion.