Finance

Those billions of people, will go with a "boom", that for sure. As to who will cause that "boom"... I'm less sure, but it seems likely that some military might have a hand. They might call them "special anti-riot operations", since "extermination" has a bad ring to it.

Supply chains are getting more automated by the day, from the robots of 20 years ago, to the AIs of 10 years ago, to the potentially AGIs of maybe right now. Supermarkets, cheap electronics, cars, produce, etc. wouldn't be needed in the same amounts with a much reduced population, meaning a much lower production could still meet demand, with no need of exploitable human labor, just some even more exploitable robot labor would be enough.

The global society we live in today, thanks to technology capable of replacing everyone, can contract a lot without actually collapsing. Maybe not all the way to "0", like some doomsayers propose, but surely a million sounds like a viable number.