This is a restatement of the "we shouldn't do anything in Space until we solve (all/most/the most important) problems on Earth" argument.
If our ancestors would have used this they'd never would have, say, colonized the Americas, or even left Africa. Because there's always something broken at home. Has there been a single moment in history that would have satisfied that argument ?
Not the OP, but I suppose one can read their comment as referring to the original colonizers who crossed the Bering strait, not those who crossed the Atlantic.
You can read as what you want, when people arrived to the Americas, the continents were covered in lush vegetation and had plenty of water, food and space.
When we'll arrive to the only planet in reach where it is even vaguely conceivable to survive for more than a couple of hours, we'll find a freezing desert with high radiation levels and no air where every cubic centimetre of living space will need to be created from scratch and maintained.
Not only that, but Mars is smaller than the Earth. Even best case for teraforming all of it into paradise and shipping half the world's population there gives ~one more population doubling time to being "full" again. Planets are not an efficient way to make surface living space with all that matter inside them.
Whatever you think of the difficulty or possibility of it, it will be easier to work out how to support 2x or 5x the current human population on Earth, compared to the effort and difficulty involved in supporting 0.001x the population (~8 million people) on Mars.
Mars is the size (surface) of the Pacific Ocean. And infinitely less habitable than the Pacific. We could settle a few billion people in entirely self-sufficient and zero-impact communities floating on the Pacific and it would be absolutely cheap and straightforward compared to doing the same on Mars.
Yet it will take only a single asteroid, massive volcanic eruption, or giant H-bomb explosion to render the planet inhospitable to all those billions of people.
Not all of them. Nuclear fallout shelters exist. A Mars base is like that, but it needs flying to Mars and landing from orbit and constructing in a spacesuit with almost no local materials and no supply chain and few people, and you can't come back when the dust settles, and that makes it orders of magnitude more expensive and difficult.
> The risk of staying on a single planet is crazy.
Unless you plan to clone yourself, you are staying on a single planet whatever happens.
If you think disaster is coming any time soon, you'd save more people trying to avert or survive it on Earth. To prioritise saving a, what, 50 person research base on Mars that doesn't exist yet over billions of people who exist right now on Earth by spending your resources on a research base on Mars instead of an asteroid detection and diversion system for Earth is crazy.
If your priority was saving the most people, Mars wouldn't be on the list. Orbital space stations are closer. The Moon is closer. Under-ocean habitats are closer. Underground habitats are closer. Self-contained arcologies are closer. (Wearing a mask is closer).
but the rewards of colonizing space are so much greater too. Imagine making gold so cheap and valueless that we now use it for consumer electronics, wiring, etc. smaller, more efficient devices, less energy loss, etc would change the world.
structural metals harvested from not earth means environmental destruction from mining would not only be unnecessary, but fiscally stupid, in orbit construction could build an equivalent loving space as your floating cities, and more, and provide theoretical backups to humanity.
There's a huge difference between sending a robot probe or small dedicated mining crew with hazard pay to an asteroid and dragging/slinging chunks of precious metals sunward, and making a self-sustaining colony on Mars.
Like the difference between building an offshore oil-rig (happens a lot) and building a self-sustaining colony on the ocean (never happens), only much moreso.
Which isn't a great surrogate because the original settlers of the Americas were not some organized exploratory group dispatched by the queen to find new lands. They were nomads hunting megafauna that were on the other side of the proverbial river when the ice melted after ten millennia.
If our ancestors would have used this they'd never would have, say, colonized the Americas, or even left Africa. Because there's always something broken at home. Has there been a single moment in history that would have satisfied that argument ?