That’s the kind of attitude that leaves you caught with your pants down in the other dozen scenarios that wipe out humanity. No one, including Elon, is saying to write off earth. It’s just within our capability now to colonize and prepare mars as a plan B.
The resources needed to colonize Mars are best suited devoted to preventing the most probable threats to humanity. A large underground bunker with nuclear power would be more viable as a plan B than a Martian colony in a time frame of the next 30-50 years or so. For Mars to be a unique contingency in the event of complete destruction of every human on earth, thousands of problems would have to be solved to make Mars viable. The engineering force to solve those problems would make Earth 100x more resistant to catastrophe.
Underground bunkers are immune to giant meteors. Also it wouldn't kill every human, it would simply just screw up most of humanity and lead to a dark age. Every mass extinction in geologic history triggered by a meteor took thousands of years to kill off the species they did. Humans would adapt over such a time frame.
please explain how a meteor would guarantee the extinction of humankind, and if so, over what time frame this would occur. I think you're overestimating the severity of a large meteoric impact on a geologic time scale. Mass extinctions occur over thousands of years. Homo sapiens' ancestors survived every one of them so far, by definition.
At a sufficient magnitude, a meteor might well liquefy the Earth's crust, or cause seismic shocks sufficient to damage even the deepest bunkers.
That said, such an event might well cause problems on a (far more generally vulnerable) Mars colony as well (e.g., ejecta subsequently impacting Mars).
And the odds of such an event are quite low. Most killer asteroids would disrupt the biosphere, and quite considerably, but not significantly rearrange the Earth's structure itself. The one time that's happened which we are reasonably confident of is the hypothesized Theia impactor, roughly the size of, erm, Mars, which is thought to have impacted the proto-Earth as the latter was forming 4.53 billion years ago, and forming the Moon as a consequence.
Other cosmic events such as a gamma ray burst or rogue star disrupting solar system orbits would have impacts affecting Mars as well, and even a well-established self-sufficient Mars colony would not be a sufficient mitigation.
(I'm generally not in the "Mars as a lifeboat from Earth" camp.)
Post-meteor earth by far. Post meteor Earth has been habitable to every ancestor of all life living today. Mars in inhabitable to almost all life except extremophiles.
