You're close to the real idea, but you've got it backwards.

Billionaires' perspective: "Gradually reducing the human population over the coming century by 99% with wars, drugs, starvation, suicide, poverty while we develop life extension technologies to preserve and ideally immortalize the most deserving (*qualifications TBD) will ultimately reduce humanity's long-term impact on the planet and be much more sustainable and leave us a lot more resources and land once we replace any jobs we might need with faster and more efficient AI and robots..."

They've completely thought this through to their conclusion already. We're the ones who haven't. They think they're simply long-term thinkers and thoughtful environmentalists doing what must be done to save the planet. What they really are is power-mad eugenocidal technocratic maniacs who place no value on human lives or the lived human experience. Other than their own. Their technological utopia is built out of utter horrors and like many utopians they have fallen so deeply in love with their dream that they will accept any cost to reach it, and they have accepted this as the cost. Therefore, they must not succeed. Utopia is an illusion. But the cruelty they will do in pursuit of their utopia is real.