No tractors. No industrial agriculture. No heavy trucks. No mining, so no EVs or renewable energy infrastructure. No shipping. No airfreight. No paved streets. No water, no canalization. No modern medicine. And so on. End of modernity, in other words.

So less than a billion people on the planet, long-term. Perhaps as low as 100 million.

It will happen, eventually. But you don't want it to happen fast.