As space enthusiasts, we often fall upon Konstantin Tsiolkovsky's quote: Earth is a cradle of humanity, but mankind cannot stay in the cradle forever.
I just saw last night a BBC documentary about leaving Earth before it becomes inhabitable as the Sun will engulf it. Robert Zubrin is interviewed.
I was quite annoyed.
This will happen in 7 billion years (after Andromeda and our galaxy have merged); our planet won't have any liquid water left long before that, in 1.1 billion years....
The human species has been around for 5 million years and every single living species has known a beginning, an evolution and an extinction. The same will happen to us, but we have no idea how and when. The human species has its own culture to rely upon or to confront when an ending will become obvious.
There simply is toooooo much time between now and 1 billion years to envisage leaving the Earth (there's no twin Earth anywhere near anyway).
Humans have been evolving on the 1 G ground of a planet with an atmosphere and an average of 15 degrees temperature.
Building artificial worlds in space (for survival) is a radical departure from our evolution (exposed to vacuum, temperature extremes, constant micrometeroites bombardments, solar flares, limited and enclosed volumes, etc..) and I"m pretty sure mankind will never tolerate an elite to do that (for the purpose of survival).
Humans will create other species capable of mass space exodus? But then, they won't be us; they'll be so different!
Earth is not just mankind's cradle...It's our home as well.