Japan has no nukes by constitutional arrangement now, but Trump might be copacetic towards that changing.
Japan could have nukes within weeks. The South Koreans, within months. They are more than capable of taking care of themselves. Times change. There was once a rationale for having large numbers of forces in Japan and SK. The SK’s didn’t have the means to provide their own security after the Korean War and we didn’t trust the Japanese to be responsible with military power after WWII. But it’s been decades since those reasons made any sense. We remain there because of inertia, and because there are plenty of people (read: big donors) getting rich off the status quo.
That is not our problem. Japan made their mess now they need to find a way to protect themselves.