We have a long way to go until everyone on the planet can grow up in conditions where you turn out 'default good/well-meaning towards others'.
Population explosion also began with agriculture, since more offspring meant more people to work your lands. Hunter-gatherers breastfed longer (so more years between ability to become pregnant again). Plus they were relatively mobile, so more having more children was not a practical goal.
Many of the diseases and other problems we have now are related to population increases in small areas (bad sanitation, etc.).
Think of it as the most extreme form of poverty. No clean water, no shoes, malnutrition, limited protection against the weather. Every day going about basic tasks carries the risk for death, disease and disablement.
There is a reason that human population didn't rapidly increase until relatively recently in our collective hundred-thousand-year history. It's because most humans died before reaching reproductive age. Even in the few pockets of the globe with conditions favorable to survival, periodic events (weather, disease, rival groups, over-hunting, etc) wiped out entire tribes in terrible ways.
I've read arguments in both directions and I'm extremely suspicious that they are almost all basically politically-driven. It goes like this:
If hunter-gatherers are predisposed to more or less gender or economic equality, or certain social structures, then that should perhaps inform how we construct our own modern societies.
In order to escape the clear appeal to nature fallacy, it then becomes necessary to argue that not only were prehistoric societies constructed in a certain way, but they were also extremely well-off. Therefore we clearly must "reject modernity, retvrn to monke", and embrace True Human Nature embodied by some cultural tradition or ideology.
The exact inverse would obviously imply that we must embrace a certain idea of technological or social progress in order to escape the "natural state of humanity" as fast as we can.
However, was the human hunter-gatherer experience was ever all that stable or predictable such that we can obviously draw out either of those major conclusions? I think a decent null hypothesis might be that all the extremes of human experience and social structure had to have occurred to some degree and that the overall average and distribution would fluctuate quite a bit over time according to weather, migration patterns, and accidents of cultural evolution that humanity at any level had basically no control over.
I find it really hard to believe that we can possibly have enough evidence in any relevant discipline to rule this out.
> The Younger Dryas (c. 12,900 to 11,700 years BP[2]) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM, c. 27,000 to 20,000 years BP). The Younger Dryas was the last stage of the Pleistocene epoch (c. 2,580,000 to 11,700 years BP) and it preceded the current, warmer Holocene epoch. The Younger Dryas was the most severe and long lasting of several interruptions to the warming of the Earth's climate...
https://en.wikipedia.org/wiki/Younger_Dryas
Everything was good for a couple of million years, and then everything got really really bad for over a thousand years.
What we think of as human history is the story of our recovery from the trauma of the YD?
Is it ethical to create a sentient AI that might suffer? -> we already make similar decisions millions of times per day
Don't treat humans as means to an end, instead treat them as ends -> we create other humans to satisfy our current needs in a world characterized by struggle
And of course as you have pointed out, some groups consider that it's better by default for a child to exist in adverse starting conditions (whilst also negatively impacting other people) than to not exist
We know the great mass of humans will carry on as before and there's little we can do to change that, but we are also aware that we have this responsibility that we can act on in our own life. Every person in the position to reproduce has a choice laid out before them. For the majority of us, it's the single most impactful ethical decision we will ever make.