The US dollar is the world's reserve currency. If everyone starts "dumping" it, what are they going to trade if for? bars of gold? bitcoin? The reality is that if the dollar becomes suddenly worthless, so does just about everything else, including your life to anyone with a gun who wants your stuff.
Yes, the USD is still the world's reserve currency. However, there has been a slow but steady downtrend starting from around 2000, mostly due to a shift to the EUR and GBP.