Nigeria is the one stand-out country that has stubbornly refused to become [much] less fertile as it developed. It's quite an outlier (was ~6.4 in 1960, up to ~6.8 in 1980, now ~5.4).
The runaway example is more of a thought experiment to illustrate why emissions per-capita, in isolation, is flawed and can still lead to devastating climate change effects. In reality, there probably won’t be countries with unlimited population growth, but India, Nigeria, and other African countries will be adding several billion people to the planet in the future decades while most of the rest of the world will be even or decline. As climate change effects become increasingly real over the next couple decades, I don’t see the rest of world being amenable to decreased standards of living while other countries add people to developed standards of living. It’s likely countries will try to coalesce around existing populations being assigned a per-capita total emissions limit for their current population. Where a country’s additional population will come at the expense of the existing country’s population’s ability to emit.
LOL at the y-axis on that graph ;)
https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location...