Because it’s not good for productive economies that can produce solutions to current problems to stagnate and die. Japan should make moves to encourage a sustainable population growth rate.
Overpopulation is a problem mostly in struggling economies.
South Australia is 70% renewables, as per their own official energy site.
Batteries are the limiting factor for renewables. Building battery storage that can supply a large city is expensive. Even the battery South Australia had Elon Musk build can only supply a town for about an hour. I’m hoping battery tech improves soon, but it seems to have stagnated for a while.