Of course sustainability is not a guaranteed outcome, it will require making good decisions, but I'm sure that a "mere" one billion people would also be unsustainable with sufficiently poor decisions.
We're in the middle of the Holocene Extinction, that some call the Anthropocene Extinction - the sixth mass extinction in the history of the Earth, and the first to be caused by a single species. Global warming has long since passed its tipping point, we're simply in a harm reduction phase right now and it's a race to see if we can even avoid the eventual collapse of the entire food chain.
The world population didn't reach one billion people until about the year 1800, and two billion by the 1920s. When I say that it can only support one billion people, I'm referring to what was possible before the Haber-Bosch process revolutionized farming and made it possible to grow enough food for 8 billion today.
I'll concede that the number might even be as high as 2 billion or so. Beyond that, we have to employ technologies that upset the ecological balance of the planet, leading to mass extinctions, loss of natural habitat, carbon emissions, and other harmful activities at a planetary scale.
https://www.issuesofsustainability.org/helpndoc-content/Club...
https://theguardian.com/environment/2012/apr/26/world-popula...
https://www.bbc.com/future/article/20220905-is-the-world-ove...