Tokyo is a fantastic city, I spent two weeks there and loved it. It is extremely clean and safe, loads of good restaurants and entertainment, and good outdoor spaces. If I could speak the language and work there, I would move in a second.

California is in no danger of being overpopulated... The issue is as you say - single family homes are a brain dead way to live. Every homeowner has to spend hours a day maintaining his house... We stopped running our own internet servers after the first ten years of the web. Why can't we do the same with housing? Convert everything to medium (4 story buildings) or high (10+ story) density housing. You would completely get rid of suburban sprawl and everyone rich and poor could live within a mile of the ocean or mountains, walk or ride a five minute train to work, and solve climate change at the same time.