I haven't said anything about "the wrong kind of people having babies" and have been pro-immigration in another comment so I don't see how this has anything to do with what I said.

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.