The counterargument is a simple opportunity cost calculation:

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.