126,000,000 people on an island nation is MORE than enough.
Japan had a population explosion in the mid-20th century and this surplus population is trying to correct itself.

In case you haven't noticed, having too many people(or any species) leads to strained resources and generally it's a waste of resources. The nation of Japan could easily be run at current output with half of its population. Especially with automation coming along. And Japan understands this. They're going to attempt a gentle transition. All they gotta do is keep out brown people, who have an objective habit of destroying civilization.