My money is currently on a combination of the two. It is not just the US-centric political power structure, but the US dietary structure as well.

The US has been very successfully exporting our animal-based diet since WWII. The planet cannot support that diet. There is not enough fresh water, nor arable land, nor fish in the sea. Deforestation for livestock grazing exacerbates the problem massively. Emissions from agriculture is huge (this report is US only).

The world population estimate for 2050 is 9.8 billion. Scientists predict at current warming rates, the planet can sustain 1 billion. I expect we will turn away from the modern western animal-based diet by necessity after the ecosystem breaks down far enough that wars for food and water combined with climate refugees forces the world to give up that diet.

My prediction is that we’ll end up at the lower end of the 1–10 billion range, and the drop in population will be painful and violent. I hope I’m wrong.