Yes but these countries also have extremely low birth rates - 1.3, 0.8, and 1.2, respectively in 2021. Japan is finally feeling the effects and has an actively shrinking population. In 2022 it lost 556,000 people. To remedy this with immigration, they'd have to do quite the about face. There also isn't an endless source of immigrants - eventually the countries that people are emigrating from will economically develop and have lower population growth. Sub-replacement fertility rates is an issue in any somewhat developed economy nowadays. It's just the worst in East Asia. Countries need to figure out how to create a quality of life that will encourage stable population.