Let's look at this way, humanity needs to become multiplanetary species to prevent extinction if some sort of cataclysm happens on Earth: nuclear war; Yellowstone erupts, etc. Compared to that, any other problem seems insignificant.
And it doesn't matter how many Dyson spheres are made, if humans aren't dedicated to improving the collective opportunity to live life without suffering, there's little real 'significance' to extending the project of humanity compared to any other life-form.
You cannot eliminate suffering.
> Life will go on in other parts of the universe, and all life, no matter where they locate themselves, will have some kind of existential threat.
There's no guarantee or evidence that there is sentient life in the universe
!= improving the collective opportunity to live life without suffering
> There's no guarantee or evidence that there is sentient life in the universe
Do you at least consider a possibility that life originated only once?
There’s a lot of evidence that points to the fact that on a global scale, we can barely care about stuff geographically adjacent to us, or worse, socially (class) adjacent to us, but here we are claiming we care about generations beyond us.
Big old lie.
The second reason to get off-world is the more far-fetched one, but assuming we can establish that other worlds are dead or sterile then we are free to customize it as we please without having to feel bad about it. In the long run we could leave Earth completely, allowing whomever comes next to develop freely without our intervention.
I'm not saying there are no existential risks at all, but being on other planets would not make us any more likely to survive to them than to simply being very well isolated on Earth (which would cost 1000x less).
Developing space technology already has all sorts of benefits and it doesn't need this poor argument in its favor.
Mars would also offer cultural redundancy. Also, sometimes people choose to do things brcause they are hard.
If I were a betting person, I'm not sure I'd go all-in on a CEO-Emperor-King of Mars over existing nation-states on Terra
I see no immediate risks to life on Earth, and considering we're quite hard to get rid of, I suspect the human race will be here for quite some time yet. The ability to do a large-scale settlement on other planetary bodies on the other hand, might go away considerably sooner, and who knows if/when we will be able to do it again if we ever end up in a situation where it might be needed.
Regardless, there are other reasons, as you say. For one, space-based manufacturing would allow us to increase the standard of living on Earth without having to worry about things like CO2 levels and pollution. If we start commercializing space then I expect colonization to follow as a natural result of that and we won't have to worry about the more philosophical side.
Having a self-sustaining outpost somewhere other than earth is valuable not only for its own sake, but also due to the developed technology and industry and infrastructure implied by its existence.
If tomorrow you suddenly need a huge fleet of fully reusable ships for some reason, the right time to start working on them was 20 years ago, or as soon as basic science and technology level allow.
There are also political risks (we are all too close to each other).