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.

And if we're talking about self-sustaining Mars colonies, we're talking about something that's relatively far off in the future. If we talk about terraforming, we're talking about something much, much further off. It's worth considering the potential future that a Mars colony would exist in. For example, it doesn't seem impossible that we'll have something like brain uploads in the next 200 years. When that happens, what's the point of trying to keep human bodies alive on Mars? It would make about as much sense as trying to use horses to ship freight across the U.S.

Or we got hit by a big asteroid.

The new manifest destiny