I think that the ultra-upper class is convinced the way forward is with less humans, way way less humans. They dream of the world as basically a huge theme park and only want enough minions around to keep the park secure, tidy and running. The less humans, the more food, the less consumption of natural resources, less trash, etc, and after 100 years like that the earth is reset into some idyllic place.
The fastest way there would be nuciear, but that would be too damaging to the planet and leave lingering radiation. The next fastest would be to go for global economic collapse leading to something akin to Mad Max. And those bands can be taken out in time.
This thought was put into my head by a guy who used to be pretty liberal and now is full blown Trumpkin thanks to Q-anon. He might have this one right, which would be the first one ever. But it does make sense. And it's doable to a certain degree, especially if you can pull it off Wizard of Oz style, so no one ever sees it coming until it's here. What would be your first move if you were them? Pit the lower classes against each other?