@browncoat - There are already creatures on our planet who don't breath oxygen and live well outside the norms of what we would consider typical life.
I think they even recently discovered organisms that live by using electricity as fuel. I mean, conceivably, you could have creatures who manage to survive in a vacuum if they can do that (with many other adaptions of course).
And the thing is, we are the source of the pollution in our air at the moment. And it's not biological. So if it gets to the point where we ourselves go extinct, the atmosphere probably won't stay the same.
Life evolves though. I don't doubt it will survive. I just think we need to consider how we want it to survive. I wish more people would look at environmentalism as a selfish endeavor. It's basically about preserving good things in the world.