This is not what the grandparent meant, and also not what anyone sane means when discussing establishing off world colonies. They are talking about an insurance policy if we happen to get hit by an asteroid, or world war III breaks out, or a devastating pandemic, or any number of other civilization (or life) ending events.
No one wants the bad thing to happen, but instead they are saying "wouldn't it be smart to have redundancy". They're also not saying "ignore trying to save the environment because we have mars". It is very likely any effort to make mars into a habitable planet would dwarf fixing problems here on earth.
They're also not suggesting spending a significant amount of our resources on Mars, to the detriment of life here on earth.