That whole romance with the idea humans need to go to space to survive... Sigh. Earth is, and will always be, the most human-friendly planet in our reach. Mars, and any other planet in our proximity, is DEAD. You have to live in airtight bunkers, go out in spacesuits. Even if our planet was hit by a catastrophe like a supervolcano, Earth would still be a friendlier environment than that. If you can build autark colonies on Mars and Venus, you can build autark colonies on a catastrophy Earth.
Then that talk about terraforming. We can't even purposely terraform our own planet.
In fact, there are several good reasons to leave the Earth.
Societies needs are exponential: While humanity is likely to level off in population at around the 10-11 billion mark (see Hans Rosling's UN data analysis) the needs of an ever-increasing economic civilization will rise, and this will tax the planet. Inherent in this, is the need for both resources and space. Space has both in abundance. Eventually, man's exponential nature will level off...BUT only because of the next paradigm..man living in space. Then a new paradigm will start the cycle over again. Humanity will then again need more of each.
Survival: Spreading out the human race to survive very real, statistically probable catastrophes has been recommended by the most intelligent people on the planet.
If we are doomed by our own hand with global warming, all the more reason to leave and hope we learned the lesson.
Philosophical level. Humanity exists in it's surroundings of space, evolved on a single planet but part of a universe that shares all of it's elements and mechanisms. The idea that we will not travel in space when our technological level allows it, seems incredibly narrow and near-sighted. We have always expanded and will continue to do so. The solar system allows for us to be there.
Far future: If the Singularity happens, or anything similar to it at any time, then eventually intelligence will spread. We or someone else will convert everything to intelligent matter, thereby quantifying the universe. The ultimate in survival may well be a way to survive an end to the universe or travel to another one..