Why should we not expand into space? To make sure that all the lifeless rocks in it stay lifeless? If earth were twice as large would you say that logically we should only inhabit half of that earth or do you believe that human beings have a natural claim to everything that can be reached via boats and legs but not via rockets? Personally, I would like a future where I can see other planets with my own eyes and dislike a future where the entire species is dependent on one planet being habitable.