Immigration won't make future generations of Americans larger than the generation before them. We're just losing ground slower than Japan. Much studied and commented on. Tl;dr immigrants have lower birth rate than 100 or 50 years ago.

https://cis.org/Report/Fertility-Among-Immigrants-and-Native...

Japan has started to encourage immigration, but probably too little, too late.

Almost every country in Asia has a declining birth rate.

https://www.statista.com/statistics/1171367/apac-total-ferti...