I think you haven't been following the news. US demographics is by far the healthiest of all halfway developed countries, with the exception of Israel. China has third low TFR in the world after South Korea and Taiwan, and is facing extinction within two generations whereas society and the economy will simply cease because all available labor will be consumed by caring hundreds of millions of extremely elderly people.

By 2100, US will look about the same as Western Europe looks today: ageing, but still OK. China will be no more.