Perhaps before you post you should do some research rather than describing anyone who doesn't want children as "selfish". We are not living in the age of Ozzie & Harriet. In many cases it now takes two incomes to support a household. Women no longer greet their working husbands with aprons on and meals waiting.
Also, young women now represent 60% of college grads so if they no longer wish to become baby-makers rather than pursuing enriching careers, good for them.
Also, do some research on the immigration rate in Japan vs. the US. They are vastly different, which is a major reason Japan is in this predicament and the US should be just fine. I'll look it up for you.
Japan's immigration rate is among the lowest in the world on a per capita basis. As of 2022, foreign nationals made up about 2% of Japan's population, which is lower than South Korea but higher than China. In contrast, the foreign-born percentage of the United States is near an all-time high of around 12%.
As for Japan's young people choosing not to have children...
Experts point to several factors behind the low birth rate. The country’s high cost of living, limited space and lack of child care support in cities make it difficult to raise children, meaning fewer couples are having kids. Urban couples are also often far from extended family who could help provide support.
https://www.cnn.com/2023/01/23/asia/japan-kishida-birth-rate-population-intl-hnk/index.html