I don’t know, it seems like planning on mitigating/averting an asteroid is a much easier engineering problem to solve compared to establishing a self-sustaining civilization on an otherwise inhospitable planet.

We definitely shouldn't give up on averting the catastrophes we know just because we've got a plan B but at the same time we shouldn't give up on a plan B just because we think we can avert a particular catastrophe.

At the end of the day "self-sustaining civilization in an already inhospitable and isolated place" is probably more about showing we've already managed to find a way to eek out under pretty much any situation than it is particularly about Mars itself. To be honest life on Mars would be significantly crappier for probably decades upon decades - but at least we'd prove out the floor and our ability to raise it over time instead of assuming we'll figure things out before our assumption where we are will always be supportive to us falls through for whatever reason.

When you don’t fund plan A, plan B becomes plan A

Earth will itself become inhospitable, assuming that we can survive on this planet as a species for so long.

Even after an asteroid impact (or major nuclear war), planet Earth would still be a much more welcoming place for human life.

Air quality on Earth would go down a bit, but there would still be air. Food supplies would be chaotic, but some crops somewhere should survive. As for severe temperature swings on a post-apocalyptic Earth, they'd probably still be easier to handle than what NASA calculates on Mars as nightly lows of -128 C.

I realize that colonizing far away places is what Earthlings do. (Columbus, Cecil Rhodes, etc.) But the lets-go-to-Mars crowd strikes me as an extreme manifestation of Dunning-Kruger

Plus Mars' atmosphere has tons of CO2. Think of the carbon credits!

Literally, but not figuratively. The atmosphere is not very dense.

Cecil Rhodes, while problematic, was born in England 200 years after the colonizing of the Dutch Cape Colony in South Africa. When he arrived in Africa, sent out for his health, in his early teens he landed on an established albeit failing cotton farm.

His wealth arose from snowballing into a monopoly the purchase of Kimberley diamond leases from other European settlers over 17 years with backers money and then his own.

For that matter Columbus was in truth a failed navigator and accidental colonizer.

His intent was sail to the already colonised spice islands of the Dutch East Indies ... but he'd made a boo boo wrt the circumference of the Earth.

Thanks for the extra context on Rhodes in South Africa, which is an accurate and valuable addition to the conversation. I'd still nominate him as an example of a rapacious colonist, based on the skirmishes, one-sided "treaties" and general land grabs that he made in the 1890s, stretching vastly farther north from Britain's initial presence in the South African Cape Colony. These fueled Britain's colonial era claims to what became known as Rhodesia (now Zimbabwe) and Northern Rhodesia (now Zambia).

Columbus ended up in 1496 as Spain's colonial administrator for what's now the Dominican Republic. He did that role badly, too, but what's more important is that he did it -- creating a colonial pathway that kept widening for centuries.

The day after the asteroid impact that killed the dinosaurs, when everything was on fire, Earth was way more habitable than Mars. Earth has the advantage that can live on it without support, after apocalypse some survivors can continue with rocks.

Mars has the problem of being self-sufficient without Earth. In the near-term, if Earth dies, Mars dies because there is something like chips or biologics that they can’t make. If we have technology to survive on Mars or make them self-sufficient with only millions, then that can be used on Earth to maintain civilization on Earth for any disaster. The money spent on Mars colony pays for a lot of bunkers.