I get your point but what about a step before that - since when is that anyone's goal? From a sociopathic leader perspective, vast populations are only great for armies and tech has surpassed the need for raw manpower at that scale (and the AI you fear would make militaries require even fewer people).

In your AI scenario is it more likely the ruling class gives everyone free living standard or just lets like 40% of the population die? If all the leaders get together this is the ideal outcome for them -- vast power and control without enough civilians to rise up, climate change becomes easy to reverse with vastly lower power and food needs, and reduced threat of global war because nobody has an occupying size army anymore. This is like the new version of "mutually assured destruction" as a strategy for global peace. I can't speak for the world but I can imagine some of the twisted folks currently in power in the US seeing this route as their destiny and simply them doing the best thing for humanity as a whole -- longtermists are in, nazis know a final solution when they see one, and Christians are honored to have the duty of bringing forth the second coming.