Taiwan population has already been in negative growth since 2020 (
https://www.statista.com/statistics/319798/taiwan-population...), not even mention Japan. Sure if everything else can be kept constant when there was no one child policy, there should be more couples that can have children now. But these population eventually will die, and the growth eventually are determined by fertility rate, which has much more to do with culture and development. The more development you have, the less likely people are having children. Confucian societies are less likely to have children than Jewish society. Muslim and mormons are more likely to have children than liberals.