Well, we are leaving, just a matter of when. When our sun red dwarfs, there's a good chance we'll survive it - just have to move the earth to a further away from the sun ( we're close to being able to do that now and we have millions of years to figure it out ). But even as a red dwarf, one day the sun will die and so will humanity if we don't get out of here. Now - if there's not such thing as interstellar travel ( i.e. not possible ), then we're screwed, there's simply no way to keep humans alive during light years of travel - maybe in the future we'll have the ability to seed other planets and there will be gaps of no living humans in the future, but that will depend on developing dependable technology that can survive years ( thousands? millions? ) of deep space travel with no accidents and no mechanized failures. If AI advances enough, maybe we can pull it off ( machines with advance abilities to address problems during deep space ). Anyways - we will be leaving some day and it's good to bring this stuff up now to assist in developing processes/agreements to handle the politics of divergent civilizations.