But that would not make any sense if one person held all the dollars. The creditors would have no use for dollars, and the US would need to switch to another cu

But that would not make any sense if one person held all the dollars. The creditors would have no use for dollars, and the US would need to switch to another currency in order to be able to pay its employees. They could print however many dollars they wanted, and could pay back the debt that way, but that would not solve the problem of the dollar no longer being useful as a currency. The government's employees would have no reason to accept the dollar because nobody else would accept it. The dollar is primarily valuable because it's used as a currency. If it stopped being used as a currency, its value would be zero. This is no different from Bitcoin.