Thanks, great discussion and great points. I agree on all of them, but the part about space sticks in my brain. While I think it's correct, imo it's only looking at one side of the long-term-human-survival coin.
>In the long view, only one lifestyle choice leads to a population that can survive the next dinosaur killing asteroid impact.
A nice future to imagine, but it's hardly a foregone conclusion. The future might lead there, or it might lead elsewhere.
In the real world it's looking like business-as-usual kills the civilization (but not the human species) before we get a chance to re-enact Deep Impact. https://www.youtube.com/watch?v=HMmChiLZZHg
Expanding into space doesn't help us if we proceed to destroy our habitats on a finite time-scale (which is all "unsustainable" means: that it cannot continue for some reason). And we haven't figured out how to avoid that in an industrial society, which is necessary for living in space. New planets don't help if every civilization you establish on them inevitably collapses. Expanding through space can't be used to maintain a non-zero exponential growth rate either, if for no other reason than that the available volume of space only grows as t^3.
Ultimately we must BOTH establish a completely sustainable, steady-state (capable of 0% growth indefinitely) global civilization AND expand into space, or perish as a species. Neither is sufficient on its own.
Agriculture does seem to help the latter, but it hurts the former. Seeing as how we haven't actually retired the asteroid risk yet, and yet we have introduced at least two new existential risks (nuclear war and environmental degradation/climate change), I'd say we're not looking that great in the long view.
Our current score seems to be two steps backwards, one [hypothetical] step forwards.