I think being a country of law, which Japan is decidedly not, is a must-have for G7 membership.
I'm not American and have never set foot in the US, so don't take my criticism of Japan as some sort of "America #1" cultural chauvinism.
The notion that a group of rich and powerful countries gets to decide how other countries should behave when said rich and powerful countries got to where they are through (both past and ongoing) atrocious exploitations of other people doesn't sound particularly convincing, don't you think?