All developed countries have falling fertility rates, most below the replacement rate. Japan used to be the poster child for the issue, now they're not even the worst example. How many tens of millions of migrants would be needed to offset the unbalanced age pyramids in Japan, China, South Korea and all the Western countries beginning to experience the same problem? How could such a massive number of people integrate into a country?

The reason people don't have kids is not just the cost of doing so but the massive commitment it entails. People have more agency and mobility than they did before and they don't want to give that up. Importing people and ignoring that is printing more money to pay off current debts.