Is it? Earth is, or was, extremely hospitable to humans. Mars is, and will be for a very long time, extremely inhospitable to humans. If we can't even take care of the Earth, what hope is there of taking care of Mars? The more likely scenario is that greedy Martians destroy their own Martian environment very quickly and make themselves extinct. It's much easier to destroy Mars (for human life) than it is to destroy Earth.
We need to focus on sustainability, otherwise Mars will also be unsustainable.
Just wait until oxygen itself becomes a commodity, like in the film "Total Recall". Except unlike in film, there's no deus ex machina native ancient technology to suddenly make the entire atmosphere breathable.
Your mistake is thinking that the plan is to move humanity away into space, instead of bringing space (or more accurately mineral-rich asteroids and solar energy) to humanity. That way we can just externalize all industry & pollution to a lagrange point somewhere and let the earth take care of healing itself.
We need to focus on sustainability, otherwise Mars will also be unsustainable.
Just wait until oxygen itself becomes a commodity, like in the film "Total Recall". Except unlike in film, there's no deus ex machina native ancient technology to suddenly make the entire atmosphere breathable.