it seems to me that being spared to be poor might be optimistic
No kidding.
Well, as far as I can tell, if each person grows their own food and carries their own water, the planet will support between 600 million and 1 billion people max.
That means about 6 billion people are about to lose a high-stakes game of musical chairs.
Curious how you arrived at those numbers. So many variables that play into it. Bad crop years, fending off neighboring people for your supplies. Access to water etc.
Speaking of water, I also wonder what role our tech pollution into the water supply might have changed long term survivability in ways that weren't an issue in times past. Out of curiosity I see (according to the chart I'm linking to) that back in the 1800's the average life expectancy was between 38-45 roughly. About half what they claim it is now.
https://mappinghistory.uoregon.edu/english/US/US39-01.html
Look at some long-term global population charts. World population has been generally flat until about the 1890's when it starts to go parabolic.
I believe average life-expectancy is a bit of a red-herring.
Yes, "medical science" and a "safe" (non-physical) working environments can keep you alive longer, but only after you pay the price they demand.
And I'm not sure how many "retirement homes" you've visited lately, but I'm not sure it's the paradise those people were hoping for (especially now).