If you'd have to recycle your water and raw materials anyways, why not do it someplace where a dome crack won't kill everybody?
Mars is appealing partly because it'll be a long time before it's crowded enough for any of that to matter.
In reality, there is no 'unused' land on Earth, because we share the planet with other living beings. Earth has a biosphere that is already severely degraded by humanity's destruction of natural habitats
I think we should go to space (in O'Neil cylinders), but I'm going to spend some time thinking about desert wasteland arcologies.
Even if mankind is quite resilient, our current society is super fragile to much smaller catastrophe, and we may actually loose the ability for inter-planetary travel easily.
We really need to diversify our location. That's not the goal of this century, and probably not even the next one, but to have some skeleton seed of civilization elsewhere than here is a logical must, and Mars is easiest and closest one. So better start doing some baby steps towards that direction immediately when we can, rather than pour trillions into more than useless warfare.
I used to think we should fix issues here on Earth before moving elsewhere, but the problem is, we won't. Its dangerously naive idea. Rather we should accept that mankind is flawed and accept and work with flaws, compared to wait for some utopia to happen here and then expand.
https://www.cambridge.org/core/journals/international-journa...
The ozone layer would then of course shrink worldwide, yes, and this would collapse a large part of photosynthesis, causing a probable food chain collapse, but we're impressively adaptable and at the very least, millions of human beings would stay alive sustaining themselves with food grown or raised under adaptive conditions. The rest of nature would also adapt based on what survives, as it always has with previous cataclysms. This would probably help us too.
In other words, humanity would suffer terribly and billions would probably die, especially since this kind of event would be so sudden and unstoppable, but we wouldn't all die and the many survivors would repopulate as the ozone layer regrows too (which would happen over a few decades since it regenerates itself from solar radiation). But yes, bad. There is a theory that a gamma ray burst caused the Permian-Triassic event, the Great Dying, and 95% of life on Earth was extinguished during that time, though much of it was much less adapted to unexpected radiation than many lifeforms on Earth are now, so things could be less severe next time.