The U.S. government, of which the fed is a part, could, if necessary, buy back all of the outstanding Treasuries in a short time. As a matter of fact, Japan bought back ~50% of their outstanding JGBs not long ago. There is an important difference between the yen and the dollar, however, and thus between JGBs and Treasuries. The dollar is the most used reserve currency for international trade, whereas to a much lesser extent the Yen is used for international trade. Currently ~58% of world trade is done in dollars, down from ~70% in the recent past. Because the dollar is so widely used as a "reserve" currency -- just means trading countries need to maintain a liquid reserve of dollar denominated assets --- U.S. treasuries are in demand around the world. Dollar reserves are held largely in the form of interest paying Treasuries rather than non-interest paying dollars deposits. Treasuries serve entirely different purposes than borrowing.
The U.S. does not borrow. That's just a myth. The U.S. has no real debt. The National debt is simply the sum of outstanding Treasury securities, but these do not represent real debt.
