Yes, well actually if the economy grows at all, a constant tax rate brings in more revenue, while we tend to spend more too. But I agree it is theoretically possible.
Except that the Fed is buying bonds that pay a higher rate that they borrow at (U.S. Treasury rate). So unless there is a lot of defaulting, they should be fine.
It's like they are mortgaging the White House at 0.25% and buying bonds that pay 4%.
Also worth looking at the last time the USA completely paid off its government debt.
> On January 8, 1835, president Andrew Jackson paid off the entire national debt, the only time in U.S. history that has been accomplished. [1]
[1] https://en.wikipedia.org/wiki/History_of_the_United_States_p...