As a matter of fact Japan has no other choice than being under US protection.

I can't imagine a chain of event that would lead the US to get out of Japan volunteerly [0], nor Japan being able to kick the US out forcibly. It's just outside of the realm of possibility right now.

[0] they won't even move out of Okinawa as the whole island loathes the US base and gives the middle finger to their own gov to get them out.