The push to get out into space would spur research into clean energy and ways to keep a biosphere running well, which will in the long run benefit Earth. Additionally, mining resources from asteroids would mean those resources needn't be dug out of Earth, which means that the devastation of strip-mining and other resource acquisition will be averted. In the end, preparing to spread to other planets may be what makes it possible to live on this one for longer.

Hawking is right about the long-term survival; eventually, whether humans cause it or not, Earth will become uninhabitable for megafauna, including humans. Spreading to space will allow humans and many other species--perhaps some of everything that lives here--to survive. Remember, if an asteroid hits Earth now, not only will there be no more humans, but also no more gorillas or whales or cheetahs or coral reefs or luna moths. Imagine if humanity, instead of being the thing that threatens the survival of all that is rare and wonderful, is instead destined to be the thing that saves it all, to go and prosper on new worlds!

Hatch, or die.

Tucker