Earlier quoted context omitted.
This is the thinking of someone on the timescales of a single life. If humanity persists another 1000 years on our current trajectory (US/world politics not withstanding), I think nothing is really a fantasy. Rather, it's all possible but maybe just not in our own lifetimes. But it is also terribly difficult for us to plan for tomorrow, let alone for a future where our descendants are at the helm.
I agree, it’s just a failure of imagination. Some folks correctly foresee not being able to continue what we’re doing now in the exact same way in some new context and conclude everything is impossible. Life isn’t this fickle, it’s adapted before and will adapt again. This is why great science fiction is so valuable, as some people are better at imagining new ways of being more than others, and can show the rest of u…
There will never, ever, ever[1] be a scenario where if you weighed up the options of "expand into some less habitable area of the Earth" versus "expand to Mars", the latter is the better option either 1) financially, or b) quality of life.
Nobody[2] ever picks the dramatically more expensive and dramatically worse option!
Also, people that are desperate enough to even consider living in the least desirable -- but still just barely habitable -- parts of planet Earth are essentially by definition too poor to afford interplanetary travel.
And no, no amount future sci-fi technology can possibly overcome the simple energy costs of this! If someone can afford the hugely energy intensive interplanetary travel, and the up-front investment required to survive incredibly harsh environments, then by definition they could more productively invest that here on Earth! It's the cheaper and better option in every possible way, and always will be.
This will remain true even if it's standing room only on the entire planetary surface -- it'll be cheaper to build levels upwards while digging downwards.
Maybe our atmosphere will become horrifically polluted? Sure, okay, air filters are faaar cheaper than a full vacuum-capable space suit!
Etc, etc, etc...
[1] Okay, fine, maybe in a million years. Whatever ends up preferring Mars at that point will no longer be "human" by any sane classification.
[2] For some values of nobody. There are morons that buy overpriced branded handbags made of literal trash. I doubt idiots like that will make for a successful, self-sustaining colony.