I absolutely hate the "but it hasn't happened yet" argument. It's unfathomably stupid, for several reasons:

1. Unless someone is certifiably mentally ill, there is a maximum number of people that a closed biological system can contain, regarding of how much magical technology we create.

And guess what?

2. We've already passed the natural organic sustainable threshold. I'm not saying "we will be overpopulated in the future." The facts say we already are.

And for rational people, birth rate declination is icy cold comfort as we had to a 10+ billion person world within our lifetime.