The easiest future with least social disruption is for all women to aim to have 3 children, 4 if they are ambitious. Why more than 2? Because many women will still have 1 or 0.
But I'm not panicked that they aren't. I don't think the downward trend is somehow unstoppable. Docos like this one https://www.youtube.com/watch?v=m2GeVG0XYTc tend to argue that it has never happened before, that no society has survived a shrinking population. It is this which I am skeptical of.
I don't think I said 10 billion was the carrying capacity. That is the predicted population maximum before it starts falling. Not my prediction. Nobody knows the carrying capacity. With innovation it could go very high. Personally I'm not excited about such a thing, even in my smaller town I think it is too crowded. Many have suggested that the Green Revolution is the main reason that the carrying capacity is higher than it was previously.