Overpopulation is currently not a problem for humanity.
For instance, when we're told we need to eat less meat, we'll it's not so much that meat production is a problem, it's that meat production for billions of people is a problem.
So all the constraining measures we're seeing either being suggested or enforced "for the environment/climate" are constraints to fit so many people on the planet.
For what it's worth, I would be delighted to live on a planet of 10 billion people who have found a way to live in a way that can persist for millenia, but we don't seem to have done so. Although, living in a well-insulated flat made with renewably-grown timber (a great carbon sink!) walking and biking distance from all of my daily needs where my meagre energy needs are met with 100% renewable energy (when I lived in San Diego I worked from home and our apartment's average load was 100 Watts so this can be done!) on a mostly-vegan diet and working 20 hours per week or less sounds pretty nice to me.
Would 15 billion people be too many? 20 billion? Are we to imagine that Earth could support an infinite number of people? Because that seems like magical thinking to me. Though a different lifestyle (vegan, low energy consumption, etc.) would probably allow the current number to get by fine.
Talking about how many people can be sustained by earth is talking about the wrong thing. It only leads and HAS only lead to the eugenetics part. Because it's always poor people in different countries who gets sterilised.
The real topic is that capitalism and consumerism is 100% unsustainable, right from the start. That is the real debate to be had. That this world is not about human well-being and happyness, but about GDP and growth, making a tiny number of people wealthy at the expense and misery of the majority.
Unfortunately this site - Hacker News - and everything related to it, Ycombinator, Paul Graham, is only fueling the capitalist game.
Why do people insist on making this an eugenics talking point?
The number of people is not the problem.
You talk about "sustainable population", there's nothing not to understand about this. Sustainable implies it's now unsustainable, but that notion isn't true.