> We absolutely have the technology to make growth sustainable
No, respectfully, we don't.
Every organism that succeeds in doing what you advocate for (growing "sustainably") is swept from the record. If life doesn't cycle (grow/shrink) or otherwise live in equilibria, life exhausts its niche on any meaningful timescale, and the universe sends it into oblivion.
Perhaps interestingly, even records of "successfully" growing are purged, because the most effective thing that persists records of life on long timescales is the descendant path of the life itself (whether that's specific DNA sequences maintained as mutational clocks by cellular machinery, or libraries of books and concepts maintained by specific civilisations), and uncapped growth collapses the informational diversity required for life to thrive and persist -- by which I mean that we, as continuously persisting living biological and cultural structures, are the best evidence of living things like us existing a million or a thousand years ago.
When the lineage dies, evidence of the experiment rapidly decays, compared to actually successful experiments that refrain from growth and collapse of their ecology. Only DNA/culture that doesn't "succeed" in growing beyond its resources survives on a significant timescale. When an overly zealous strain of life grows too much and fails, evidence of that life is swiftly and rapidly purged as well, for any later life that cares to try to look.
I don't why biological limitations restrict humanity's long-term future.
We are already ignoring them - right now it's freezing outside, and no human could not survive in such weather using biology alone. I am also living in a city which is way too dense to sustain natural human society.
There is always a risk of entire species dying out, for example via global war, or everyone suddenly deciding they don't want technology and then freezing to death in the winter; but there are no universal long-term limitations. At some moment there will be independent colonies at other worlds, and then humanity will be eternal.
No, respectfully, we don't.
Every organism that succeeds in doing what you advocate for (growing "sustainably") is swept from the record. If life doesn't cycle (grow/shrink) or otherwise live in equilibria, life exhausts its niche on any meaningful timescale, and the universe sends it into oblivion.
Perhaps interestingly, even records of "successfully" growing are purged, because the most effective thing that persists records of life on long timescales is the descendant path of the life itself (whether that's specific DNA sequences maintained as mutational clocks by cellular machinery, or libraries of books and concepts maintained by specific civilisations), and uncapped growth collapses the informational diversity required for life to thrive and persist -- by which I mean that we, as continuously persisting living biological and cultural structures, are the best evidence of living things like us existing a million or a thousand years ago.
When the lineage dies, evidence of the experiment rapidly decays, compared to actually successful experiments that refrain from growth and collapse of their ecology. Only DNA/culture that doesn't "succeed" in growing beyond its resources survives on a significant timescale. When an overly zealous strain of life grows too much and fails, evidence of that life is swiftly and rapidly purged as well, for any later life that cares to try to look.