I like the setup for the Expanse, because it paints a very plausibly realistic future of how the movement to a solar species could turn out. Earth has the air humans need everywhere, and will always remain the birthplace of humanity and where to buy essentials for human life from. Mars has the potential to be turned back into a livable planet with enough scientific encouragement. The belt and outer planets is a source for ice(=water/air) and other useful materials both for sale to the inner planets, but essential for the outer regions, and the people here would naturally become a working class people. All in all it is a sad, but also fascinatingly plausible turn of events once beyond the moon is populated.