> Given this is true, then there's no particular reason to think that even 100% reduction of emissions would meet any goal.

There is, but it’s been outside the scope of this discussion.

The exact reduction needed depends on what level of damage your are willing to accept, but 99.9% reduction relative to current emissions is basically the threshold for maintaining static CO2 levels until all the fossil fuels are burned. This still isn’t amazing at current CO2 levels, but as it’s the (approximate) steady-state emissions level, it remains a useful approximation of maximum safe emissions regardless of what the actual concentration is.

More emissions? The concentration drifts upwards from whatever is deemed to be acceptable when you started.

> Here's a misunderstanding between us. I take for granted that "willing" isn't a factor, that 1/10th of the population would be unable to maintain current technology, that most, although not all, of technological advancement has come from increased population allowing more specialization.

We definitely have a misunderstanding, because I’m afraid I don’t follow your argument, and I sense I’m not even aware of what conclusions you’re trying to reach with them.

I will attempt to respond nevertheless.

If we go back to the pre-industrial age, even if you get near perfect buy-in from everyone alive today, nobody is going to think it was a good idea in retrospect, and in 80 years everyone will ignore any pleas from the ancestors in the history books to never return to all we have now. We just go through a hybrid of the Industrial Revolution and the renaissance, which sounds like an awesome setting for a novel but not actually that good for the environment.

By the time the new societies have enough spare capacity to fix what we can already fix, let alone the stuff we can’t, CO2 levels will be much much worse.

> It seems like something is missing in the previous paragraph, why do 50 million people have something different, and what is it?

I was focusing on the negatives explicitly to show why it won’t be accepted by basically anyone if you try this. To focus instead on what the rich will have:

“What” the 50 million have is business, property, pets (rather than livestock), and some limited travel. Might even be able to dig up and reuse old plastic etc. from the before times, but (returning to the negative because I don’t think I’ve fully emphasised how much this will suck) even for the rich, many things trivial in our society simply can’t be newly made at all in the scenario you’re describing, as that is some combination of unacceptable (if you allow modern fossil fuel power stations, the pollution per person is too high, so no aluminium) or simply impossible without ending the scenario (can’t develop a vaccine for a novel virus on a supercomputer because the world economy can’t afford to build the machines that build the machines that build the parts for the supercomputer, but even if it could, the tantalum mine needed for one specific component is both closed and probably in a different continent which is now only accessible by wooden boats and you don’t have any advance warning of Atlantic hurricanes forming).

The “why” is that this was what happened in every historical society of this type that I’m aware of. The rich 50 million is everyone who isn’t literally (in the medieval feudalism sense of the word) a peasant — not a very high bar by modern standards, but not passable without tech that one way or another breaks the scenario.