Why? Like all declines of civilizations, it's a complex mix of cultural, economic, political, and environmental reasons. And thus beyond the scope of an HN comment.

It won't all depopulate. No, but I wouldn't be shocked to see claims over Okinawa (which is not depopulated). No, but I wouldn't be surprised if the U.S. is eventually allowed to consolidate its bases into one in Okinawa.