To add to that, even if we can technically support a lot more people with the sustainable resources we have, which I personally doubt, getting close to our max capacity means we will have zero wiggle room when things go wrong. With climate change accelerating and guaranteed to cause drastic changes to our environment, a lot of things will go wrong at the same time.

The analogy I like is your income/spending ratio. Yes, you can technically spend 100% of your income, or even >100% with debt. But if you choose to live like that, then when you get laid off, or get sick/disabled, or divorced, or forced to start caring for a relative, or have your house burn down, then all of a sudden what's already a major emergency is exacerbated by financial issues.

Decreasing our population now is like increasing our savings rate. We know difficult times are coming. Fewer people will only help us navigate the transition more humanely and equitably. Not only is this true from a macro/global scale, but also a micro/family scale where it's obvious that having more kids than you can afford or have time to raise is its own kind of cruelty.

Anyway, if you're choosing to have kids, one is enough. The real winners adopt.