The assumption is at current interest rate which is a approximation. At what rate the bonds were accumulated is not taken into account. Any further debt addition will be at current yield.
The debt stood at $9T in 2009. And in 10 years US has added $20T debt. If the yield becomes 5.3 in another 10 years ???. I am not talking about the trade ties with china. But just the balooning trade deficit with China. Then China is the major trade deficit. Then we have Japan. Arabia, India, EU. Is there any country that US has a trade surplus. So the idea is spend to mend, will mend, China, Japan, Arabia, India, EU. The Reaganomics worked fantastically in 1980 because US had GE,IBM, Digital, Boeing, Caterpillar, Jhonson&Jhonson manufacturing goods locally and had a trade surplus. Atleast not a huge trade deficit just like today. That is why the Bush era tax cuts from 2000 not only failed but pushed US into mountains of debt. Reaganomics was a stellar success from 1980 till 2000. And US had a surplus budget in year 2000. India opened up in 1994. China entered WTO in 2001. They both picked up steam with tax cut in USA. China has lend USA more than a trillion. India a quarter trillion. Both countries were bankrupt in 1990.