I'm a little baffled by all the edgelord doomerism here.
Lets get the most important question out of the way. Is it possible to live in space long term? Well yeah, many people have done so, some for more than a year. And they seem to be relatively fine for it. Theres some health and economic issues sure but there is no particular reason to believe these are completely unsolvable anymore than whatever issues Columbus had with his trip. Now with that out of the way the question should be what's going to keep everybody on Earth forever and ever? Many some doomsday scenario that wipes out the human race before it could get a big enough presence in space but other than that.....
I'm fairly sure some of the same people scoffing at space colonization would be scoffing at the idea of airplanes.
It isn't the same thing. LEO living and living in deeper space are two different entirely sets of living conditions, primarily due to Galactic Radiation (outside LEO). No one is disputing LEO living or space stations for temporary living, but there is no one - and I mean no one - who thinks Humans can live well in any form of space for more than just a few years without severe side effects unless you change what humans essentially are. We can't conflate LEO living with a "space faring" civilization.
But that aside, even if Earth was to be blanketed in Radiation from a Nuclear War, it would still be easier to survive on Earth than it would be Mars. That is how intolerable the planet is. No one isn't saying you could never live on Mars, but Science fiction has skewed the reality of what it would take and has the populace believing a nice metal bubble on Mars would suffice and it would not. It would require incredible infrastructure to be transported to the surface of Mars - even when using Mars own limited resources - to survive.
When the numbers are crunched, the cost to have anyone living on Mars - even if you scale it up for economies of scale, is costing Billions of dollars per human. The idea that spending Trillions of dollars on putting humans on Mars for the "Plan B" of Earth is ridiculous, it would be cheaper just to build bunkers here on Earth, or in LEO that would be far better, cost far less and have easy access back to Earth.
Please remember this: We have had the technology to go to Mars for some time, there was even several models of Saturn V's with modifications designed expressly for this purpose, and the technology doesn't have to be reinvented to go to Mars or land on the surface, what is prohibitive is asking citizens to pay for a vanity project that doesn't particularly give you anything back other than a good fuzzy feeling. We've also had the technology for living on the Moon and in LEO for decades. But transporting 8,000 tons of lead up into space for shielding for a small station outside of LEO is, well, very expensive. Humans are cheaper and can be rotated. We can create a centrifugal force driven space station, but that will require (at a guess) 3,000 tons of equipment to do it properly for a small one. That technology has been around for decades. Again, the price ticket for such is immense. Humans are cheaper. Rotate them.
We aren't talking millions to go to Mars, we are talking hundreds of Billions. And when we start talking about even a temporary outpost we are talking over a Trillion, and when we talk about enough infrastructure for 10-50 people we are talking enough money to essentially solve World Hunger or clean out air and oceans from pollution. And we can't even get wealthy nations to pay for that right now, and never will. And when we talk about Mars colonization, we are now talking about stripping Earth of significant resources that's going to destroy millions of lives here for a few people on Mars.
To get 100,000 tons of material to Mar's surface (which would be barely enough for a decent outpost), nothing is going to change the Tyranny of the Rocket Equation, and it means several thousands of starship launches into LEO and from LEO to Mars. Consider how much energy and resources that actually is, and the bill.
All good for space shit in LEO, and yeah, a moon base, and yeah a hotel for the rich and famous and yeah maybe some lunar mining for Helium 3, some space stations for research sure...that's where it realistically ends for this century.
There is no disaster scenario that results in the natural resources of Earth being destroyed entirely or the atmosphere being made entirely unusable that are realistic outside of a 100 Mile Asteroid, and even then, after a century Earth could still be repopulated easier than Mars ever will be. Ever.
Space will be valuable for mining, for satellites, and research stations and technology that requires 0 gravity and no atmosphere, so there is money in space infrastructure and tourism even - there is no question of this. But no one is living a mile underground on Earth because it contains minerals and money, and no one ever would, and Space is pretty much the same thing - nothingness.
We travelled to the top of Mt. Everest, the Moon, the Mariana Trench and the South Pole all because it was an incredible feat - no one lives in any of those places and never will, and they are all FAR easier to live at than Mars.
And as a reminder, astronauts are elected not just for their brains, they are selected for their ability and plasticity to survive in shit conditions for long periods of time and be happy about it. Space is living in a tin can, shitting in zero gravity, owning virtually nothing, amusing yourself, having to exercise constantly to avoid bone mass loss, eating shit food, recycling every drop of water and loving the same room every day because nothing ever changes. The only environment on Earth similar to the annoyances of space is prison, and even then, they live better than an astronaut. Or being an X-Box gamer in a basement.
And Mars would be much worse.
