well that's exactly the point - apparently our population needs to keep growing forever "just because". But if any other animal had no bound on the upper limit of their population the entire globe would be taken over by it.

I like people! I have two kids myself and love them. But it's not like life was horrible when Earth had 5 billion people.

(Note - I am NOT saying that people in poor countries, who tend to use a minute fraction of the resources of rich people, having lots of kids, is necessarily an issue. But if we want to make a higher standard of living possible for everyone in the world it will be easier to do, ecologically speaking, with 9 billion people than with 15 billion)

How about «enough to have a supply of extremely talented people, able to devise novel solutions to crippling long-term problems we face»?

You won't have many brilliant people in a small natural population.

Without them it's «not with a bang, but with a whimper»

To spend most of their lives working for others.