I remember some discussion of this in Kim Stanley Robinson's "Green Mars", IIRC: he points out that it's simply too energy-inefficient to ship a few billion hum

I remember some discussion of this in Kim Stanley Robinson's "Green Mars", IIRC: he points out that it's simply too energy-inefficient to ship a few billion human beings out of Earth's gravity well, and no extraterrestrial habitat will have an ecosphere anything the size of Earth's. Earth is simply the best place to support humans in the galaxy, and it needs to be sustainable in its own terms. We can and should live elsewhere, but that doesn't mean Earth will be any less important to us.