A thermonuclear war and the ensuing nuclear winter would shift the situation quite a bit. Which is not that far-fetched, we have the capability and have been dangerously close a couple of times. Earth might still have stronger fundamentals, but it is good to have options. And, perhaps more importantly, technology developed in pursuit of becoming a multi-planetary species might very well help here on earth too - both in our current development path and in case of disaster.