Growth doesn’t matter as long as people still treat the currency like it has value. Most of that debt is owed the the people of the United States, and as long as the US govt continues to pay interest on T bonds and make Social Security payments people will continue to have confidence in the system.

Take a look at Japan as a more advanced example of this, with a larger debt as a percentage of GDP than the US and still maintainjng a sustainable economy despite population shrinkage