There’s an assumption in that statement… 
Many many companies have discontinued pension plans as unsustainable.
The problem is that in terms of balance, government debt is very much like household debt. Keynes said that governments could spend money when times were tough to help the economy (roughly). Politicians have taken that to mean that deficit spending is OK - but ignore the proviso “when times are tough”. Times are always tough. Like the household, sometime debt is good - whether the family buys a house, or the government builds a bridge or sets up a university - that is spending intended to improve the overall outlook and so is good debt.
However, when the government has to borrow just to make the operating budget, that’s like the family borrowing to buy groceries. If you can’t afford it, something has to give. It might be justifiable to borrow money to buy groceries while a breadwinner is sick, and once he/she is well again, they have a plan to get out of debt. Meanwhile, there’s enough coming in (sick leave pay, or from other spouse?) to make the minimum payments on the credit cards so nothing is foreclosed.
The government has a plan to get out of debt too, it’s called “the diet starts tomorrow!” The cuts to spending will always start after the current politicians leave office. It’s a moving target.
the short answer to the OP is “you are right, it is not sustainable”. However, at this point, the cost of debt as a percent of budget is quite low and quite sustainable. It just can’t go on forever. Part of the problem is that as the debt grows, or as the economy changes/improves and the feds have to fight other borrowers, interest rates will go up. What’s sustainable debt at 3% may not be at 9%. Plus, each year of additional deficit/debt adds to the total.
The other problem, as alluded to, is that it depends on what currency. The USA could reduce its debt burden by printing money (which bring its own set of consequences, mainly inflation and higher interest rates). States, cities, and Greece (Euro countries, that is) cannot. By printing money, and allowing inflation, a country effectively cuts the cost of its obligations too. If the USA committed to pay all its retirees $1000 a month, but after inflation everyone’s wages doubled, so taxes did too, but retirees on fixed income stayed at $1000 - well, costs have been cut but at whose expense? The people who bought a million-dollar T-bill will be able to buy half as much when it comes due… but if the US wants to sell another million T-Bill, they’ll be paying a much higher interet rate just in case. Or they’d ask for their T-bills to be denominated in something more stable like Euros (ha, ha) or Swiss Francs.
Printing money, like borrowing, can be a slippery slope to disaster.
(the US states and cities, and Greece, for example, can’t print money -so they go bankrupt, pay their bond holders pennies on the dollar, and the banks suffer… and the economy where the banks are grinds to a halt from bank failures… meanwhile, pensioners get nothing, civil servants go unpaid, local shops close because nobody ha money, etc.)