Most likely the government will takeover the job of increasing population by themselves like running a boarding school which kids are born by surrogate mother and train up school manner.I read before that Japan once considered taxing those without any child. This is to internalize the cost of not having a child. After all from a tax perspective, families with kids (future taxpayers/workforce) are supporting those without any kids in their old age.
But of course it's just an idea which is unlikely to pass in a electoral democracy.
I wonder if China could pull it off.
Lol if you think East Asia is going to allow large scale migration from South Asia with very different skin colour and culture. They would rather shrink to irrelevance and let the economy tank.