BigNick
Thinks s/he gets paid by the post
But even then, only up to a point. If people lose confidence in the US economy or leadership then they will stop buying (or sell) their US debt.French officials in the 1960s called this the U.S.'s "exorbitant privilege": the U.S. can borrow in its own currency, unlike many other countries. If a country like Argentina borrows in dollars, it risks default if it can't earn enough dollars. But the U.S. can just issue more.
Remember the "PIIGS" countries circa 2009 (Portugal, Ireland, Italy, Greece, Spain), how everyone laughed at their crazy high bond yields and called them economic basket cases? Right now their yields are all lower than the US's. And the dollar is down by about 8% against the Euro in the last month (a bit less against other currencies), which means that all products imported from Europe will cost more (unless the overseas supplier wants to eat the difference) even before tariffs are added.