What about we start by not destructing our planet? "No no but look: we destroyed most life on a whole planet, but it will be different in a spaceship: there we will live happily in peace forever, without any problem we can't handle (because we destroyed them with our habitat)"
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.
I am of those who believe that if technology is a big part of the problem, then more of it won't make it better. But it's not like I have a choice, so instead of waiting for a potential asteroid that will kill me while some (rich US citizen, maybe?) survive on Mars, I am here waiting for the catastrophes we are creating.