Once we have figured out how to achieve homeostasis on a planet so plentiful in resources as our own, then we could start looking to do so elsewhere in less favorable environments.
Mars is extremely barren and has negligible capabilities for life support.
The biggest issue I can think of is oil running out, but we're on a good trajectory to solving that. Other resources seem either abundant or replaceable. The threat of global nuclear war is problematic but not really solvable by throwing more people at the problem. Overpopulation isn't projected to be a problem. Climate change will be a major inconvenience and costly, but unlikely to be an existential issue.
Of course there's lots of injustice, hunger, disease, murder and torture that would be nice to resolve, but it's not an existential issue for humanity.
I don't think we have any issues like that as a species, as a species we are pretty much thriving. That growth might not be built on the most sustainable principles, but we are slowly getting there.
Short of something super apocalyptic ruining the whole planet for most life (like a big asteroid hitting us), I don't see humanity eradicating itself completely anytime soon, we are quite a sturdy bunch.
> Once we have figured out how to achieve homeostasis on a planet so plentiful in resources as our own
We've had plenty enough time trying to do that, maybe it's time to try a different exercise with more constraints to motivate creativity? Mars could be exactly that.
Between environmental disaster and global warming that could wipe a huge part of humanity, the ever present possibility of a nuclear war, tolerance to antibiotics, and other such niceties, I can't even begin to see how would one think that...
That's the point there: We can easily wipe out huge parts of humanity. Huge parts yes, but the whole species? I don't think that's gonna happen unless something of a truly apocalyptic scale happens, and as ingenious we are, I doubt we are that ingenious to make that happen any time soon.
Note: I'm not saying everything will be just fine, I'm just saying we are a rather resilient species as we don't need actually that much just to "survive".
Despite what you hear about electric cars and so on, every year the human race increases the amount of environmental damage it does compared to the year previous. We're not just increasing the damage; we're increasing the rate of increase of the damage. Heck, not just the first and second but also the third derivatives are all positive.
So in fact we're not getting to sustainability at all; we're moving away from it faster each year. We're accelerating into the apocalypse. Some day that might change, but not this year. First we'd have to stop accelerating. Then we've have to slow down. Then we'd have to start reversing.
There are ~15,000 nuclear weapons deployed right now by 9 nation states. That's enough to wipe out almost all of the world population, and I'm not convinced the few that remain would survive in the food scarce, irradiated lands for long.