If things on earth get to the point where we need a "backup earth" I'm pretty sure one of these things will be true

1) If we can live on mars, we can live on whatever state earth would be in, even if it was mars-like.

2) If earth becomes unlivable, the exhaustion of resources would mean we'll never be able to get to mars in the first place.

Either way, I just don't see a mars colony happening as anything except a temporary experiment. It's a really hostile environment.