Well I’m going to agree with you. That’s precisely my point too, which I’ve been arguing for some years now and also written up in several kindle books. In many discussions, I’ve never had a satisfactory answer. Nor have I seen anything in the books or papers that is a satisfactory answer.

Also - why rebuild civilization on Mars? That’s like - suppose something devastating happens on Earth, say mass famine, illness or whatever. So you decide to rebuild civilization in Antarctica or on the sea floor!

(I’ve got lots of detail to follow up more in the books, see the end of this answer, so won’t provide links here. This is like a summary of some of the ideas).

Mars is far more uninhabitable than either of those places, it’s like living on a desert raised to a height of 30 km several times the height of Everest, so high that you need a full body pressurized spacesuit to survive - and even that would be far more habitable than Mars.

Surely we’d rebuild in places with abundant water, not too hot and not too cold, with air to breathe just available by opening a window, protection from solar storms and cosmic radiation, no need to hold in the atmosphere against an outwards pressure of tons per square meter against a vacuum. In other words, exactly the same places we are living now. We evolved here and Earth is ideal for us and there is nowhere in the solar system that’s remotely as ideal as a place for humans to live as Earth.

If anything went wrong, well the Mars colony, dependent on technology more than anyone else would be first to fail. If it is some issue caused by technology - well again as our most high tech society, it would be as likely to start in their colony as anywhere else. Then there’s the risk of life native to Mars returned to Earth - probably just microbes but you can’t show that microbes with a different biology would be safe to return to Earth.

The idea that a Mars colony is safer than an Earth colony because of natural quarantine doesn’t work - because it’s easy to set up a quarantine on Earth if that’s what is needed. Okay it would take six months to get to Mars right now, but as transport improves, it will probably get down to weeks and even days. There are ideas on the drawing board that could make it much faster to get there from Earth.

Terraforming is not practical - lots to go wrong, it would take a thousand years to end up with a planet which would just about reach the stage where trees can grow there - but no animals or birds or insects - and humans able to go around with just air breathers - but with a scuba diving type oxygen nitrogen closed system as the carbon dioxide in the atmosphere would be toxic to us (it is poisonous to humans above 1% which is not that high a concentration).

But that is very optimistic. It probably doesn’t have enough carbon dioxide to do that according to latest estimates of its dry ice reserves. And if it can “terraform” as quickly as that - well probably it can unterraform quickly too. On Earth it took millions of years, and Mars is so different from Earth that even if we could magically transport the Earth’s atmosphere to Mars, it would be too cold for trees to grow there, because it is so far away. So even if you had a thick atmosphere, you need greenhouse gases - not just a bit, but cubic kilometers mined a century - or orbital thin film mirrors similar in size to the planet, to keep it warm enough to be habitable. Indeed it’s rather a mystery, how Mars could have been warm enough for lakes and seas in the past. Perhaps methane, sulfur dioxide, and other natural greenhouse gases were involved, or maybe the lakes and seas were liquid at times when its orbits were more eccentric so that it gets briefly close to the sun every two years.

Maybe some time we can do megascale planetary engineering with those vast mirrors or the hundreds of nuclear power stations and vast mining plants on Mars to produce greenhouse gases - and redirect comets in vast numbers to impact on Mars to create an atmosphere and deliver water to the planet - the authors of the papers on Mars terraforming suggest we could do it, rather optimistically. But if we could do that, why would we build such megastructures to try to terraform Mars?

If we can do huge construction projects like that, why not use them to bring low cost abundant solar power to Earth, or to build free floating space habitats anywhere in the solar system kilometer scale, spinning for artificial gravity and designed for an ideal climate inside (whatever you like, tropical, temperate etc)? Or why not make the lunar caves habitable (they may be kilometers in diameter)?

And in a future where we can make planet sized mirrors to warm up Mars and redirect all those comets, we surely know the position of every rock down to sub meter scale in the entire inner solar system and larger ones right out to Pluto - we won’t be at any danger from asteroid or comet impacts by then.

And anyway - there are no natural disasters that can make us all extinct. We’d survive even a giant asteroid impact of 10 kilometers in diameter or larger - a versatile tool using omnivore like ourselves, there’d be millions of survivors at minimum. But we already know of all the 10 km plus asteroids that do regular flybys of Earth and none hit us in the next few centuries, and comets that size are too rare to consider at present, at least going by the ratio of comets to asteroids in the objects that fly past Earth (less than a 1 in 100 million chance of one of those per century). Supernovae and gamma ray bursts seemed plausible until a decade or two ago, but now we know our neighbourhood out to hundreds of light years, detect even objects as faint as isolated brown dwarfs out to more than ten light years.

It’s just no longer possible that there’s a potential supernova so close as to be devastating. We know of many potential nearby supernovae that should explode over the next few million years, but they are all far too far away to be of concern. Also, the only known potential gamma ray burst that is close enough to be of concern, as it turns out, is tilted tens of degrees away from us and is also rather distant. Plus there is now a lot of skepticism about whether gamma ray bursts can cause mass extinctions. And again none of these would be devastating to the planet to the extent they could make a versatile tool using omnivore like ourselves extinct.

If you find this unconvincing - you have to compare having a colony on Mars with a backup colony on Earth, not with having no backup. Earth’s population is its own backup because the Earth is so vast and there is no disaster that could affect the entire Earth. If you think a backup is needed, it’s far far easier to build it on Earth. Now if our solar system was in the galactic core and regularly hit by other planets and stars, then it would be different. But there is no possible event that could make Earth uninhabitable to humans, or remotely as uninhabitable as Mars. So if anything happens, the place to rebuild is always going to be Earth.

Now, if you are a science fiction writer and you can load the dice and set up a scenario precisely as you like to further the plot, you can probably make up a scenario where it helps to have humans on Mars.

But for each sci. fi. scenario you could invent where Mars “saves the day”, there are probably several other sci. fi. scenarios where humans on Mars are actually the original cause for disaster on Earth. E.g. wars between space colonists and Earth - or else, things like they develop self replicating nanomachines and let them loose on Earth (many decades into the future I think but might some day be possible).

So - my answer is just that they rely on fantasy as far as I can tell. They have artist’s impressions of people zooming around on Mars in futuristic vehicles, with an Earth like landscape, and say “look we could do this!”. But none of this is remotely practical, not for our current technology.

Maybe it could be done over a period of thousands of years, but then again maybe not, or not with our present technology. It is just so beyond anything we have ever done or had any experience of. Perhaps after first building colonies in lunar caves, and building solar satellites to beam back terrawats of power back to Earth - and mining the asteroids and the Moon - and with lots of experience of doing closed system habitats in space - perhaps after all that we will know enough to look into whether it is possible or not.

Right now, it’s just an idea. I’m all for people writing articles and papers and studies on “terraforming” Mars. But we haven’t yet built our first off-world self contained colony growing all its own food. We could do that either in orbit or on the Moon. Before we do that, it is just way premature even to think about it.

And - no, I don’t think this will add anything to our security. The Earth is where we make our stand, whether you like it or not. Earth is still very habitable. There is nothing we know of that could make it anything like as uninhabitable as Mars. We’d have to get rid of not only all the oceans somehow, and most of the air but also just about all the ice at the poles too, move it out to the orbit of Mars, stop its magnetic field somehow, reduce it’s mass to a fraction of what it was, and it would then begin to be as uninhabitable as Mars. That just ain’t going to happen, we do have rather awesome power compared to our ancestors, but we don’t have those abilities.

Now we can do lots of useful things in space. I think mining in space from asteroids or the Moon could well be useful, seems to work in theory though not yet proven in practice of course. I think humans in space are a natural thing to explore, but in an open ended way - if we have humans in closed system habitats then they only need to be resupplied every few years which would reduce the cost of space exploration hugely. But we can do all that on the Moon as the far more natural place to do it.

Meanwhile our focus has to be on helping Earth. Any idea we can “run away from Earth to a new planet” is just fantasy and won’t solve our problems here or give us more security. Rather, if it is done in that spirit it’s just going to distract from the real issues we can deal with. But humans in space done with Earth as priority, knowledge, resources, and so on - that could be very beneficial to us. And we could do a “backup” on the Moon - the most stable place nearby, build underground seed bank and knowledge library which would still be there millions of years into the future. Even build it so it can be interrogated from Earth via radio to help any future civilizations that have forgotten space technology and can’t yet visit the Moon in person.

Maybe Mars will prove to be of value at some point for humans. But right now I think it is of far more interest as a place for scientific study. Not even mining, we know of nothing valuable enough to mine on Mars to return to Earth. And it’s of most interest for the discoveries we might make about exobiology. Either it has life or had life, or it didn’t. Either way we can learn a huge amount about a type of planet that may be very common in our galaxy. Commonplace though it may be - still - this is the only Mars like planet in our solar system. I think our first priority is to not introduce Earth microbes to it, until we are able to study it carefully either from orbit around Mars or from Earth. We don’t know what the effect would be of introducing new life to another planet, and especially a crash of a human occupied spaceship on Mars would be pretty much an immediate end of all our current efforts to protect it from Earth life.

See also my books on kindle and online:

• Case For Moon First - also covers ideas of a backup on the Moon and the many advantages of the Moon over Mars for humans - wins over Mars in just about every comparison.
• Why Humans on Mars Right Now are Bad for Science. Includes: Astronaut gardener on the Moon
• OK to Touch Mars? Europa? Enceladus? Or a Tale of Missteps?