Lots of good reasons to establish a space colony, fleeing the earth is not one of them. By this sort of argument you actually discredit the idea of space exploration.
Why not? If a big asteroid is going to hit earth (for example), fleeing is a pretty reasonable response. It's also a reasonable defense against technobiological hazards.
The impact would have to be absolutely massive to cause enough damage to the Earth that it is less liveable than Mars.
To compare, the Triassic–Jurassic extinction event created an Earth still more liveable than Mars. Even if we go way back in time, to when there was no land-life on Earth and a poorly oxygenated atmosphere, the Earth is still better off than Mars for supporting human life.
If the goal is to save the human race, then it'd be better to have a few high orbit or lunar habitats, then if something wipes out the majority of civilisation on Earth, we immediately begin recolonisation of our home planet.
Let me ask you this question: Would you consider the Titanic, oh, I don't know -- a success story?
Because that's basically what this sort of thing comes down to, at the end of it. There's basically nothing that is going to be existentially, immediately threatening to earth on a timescale that Mars is actually a more livable environment.
Where else in the Solar System could we even get the amount of water we would need?
The chance that an asteroid hitting Earth that actually matters is extremely low. It is even lower chance that we can detect it on time. It is much higher chance that politicians will start a nuclear war that wipes out most of humanity from the face of Earth.
I'm not certain what you mean by "fleeing", but we sure as hell aren't going to move seven billion people. A bunch of engineers and rich people may be able to get out, but that's about it. To save the rest, we'll need cheaper methods.
Why not? If a big asteroid is going to hit earth (for example), fleeing is a pretty reasonable response. It's also a reasonable defense against technobiological hazards.