Obviously you run into a lot of other issues with a sudden change but we are frankly pretty overpopulated and could use a decrease. It turns out that a system predicated on infinite population growth with finite resources can't be sustained forever. Especially with how finite they are in our case.

I think even worse than our population issues is the overcentralization though. SO much is based around Tokyo, the vast majority of jobs are located there, especially if you want any upwards mobility. It's even worse if you include areas like Chiba and Saitama as part of the greater Tokyo area. Even the other cities pale in comparison to opportunities in Tokyo. If people could spread out a bit more, it wouldn't be nearly as bad as it is now.

I was lucky enough to be able to move out of Tokyo last year thanks to my work. I am not in a super rural area but I definitely never want to go back to Tokyo if I can help it. I am in the mountains around Mt. Fuji, and get to work overlooking a spectacular view of the mountain. Don't have to deal with packing into crowded trains in the summer when everyone is dripping with sweat.