Those countries weren't buying dollars, they were gaining them in trade. Maybe a better way of looking at it was that various countries were accepting dollars

Those countries weren't buying dollars, they were gaining them in trade. Maybe a better way of looking at it was that various countries were accepting dollars from the US for goods of various kinds. In other words they were accepting colored pieces of paper in exchange for real things like cheeses or cars, or aircraft. So when they wanted to reduce the number of dollars they were holding on hand, they went to exchange those excess dollars for gold. Nixon refused. He had to, otherwise the US would have been forced to default on its debts. From then onwards, the US Dollar has lost roughly 98% of its value in those 50 years. Gold used to be worth $35 an ounce till Nixon's default. An ounce of gold is now just on $2000. $35 as a percentage of $2000 is 1.75%, so the USD has lost 98.25% of its value. A Rolex Oyster watch in 1970 was $210 dollars, its cost is now somewhere around $10,000.