Actually, they are “worth” what they were when they were bought. The ‘present value’ is lower because interest rates have gone up, but if they hold to maturity, at the moment there is no suggestion the U.S. will default on paying back the bonds.
My treasury funds went up quite a bit over the previous months, and have gone down recently, because of the changes in underlying interest rates.
We must hope that the Chinese and others holding these bonds believe that they will be redeemed in 30 years at face value in terms in present day dollars. To believe that you must be living in “Alice in Wonderland.” We are going to default either in terms of gradual or maybe rapid decline in dollar value or simply partial repudiation of US debt. If I had to choose one over the other it would be partial repudiation. There is no experience worse than constant and unremitting decline in the value of a currency. It makes paupers of those who saved and leads to the degeneracy of a society.