We’re way off topic here but what the heck… in my entire life, I’ve not seen one single ‘population model’ play out accurately. We’ve seen dozens of popular ones that have failed to come true in the past century alone - what makes today’s different?
China is currently set to face a reversal somewhere from the middle of this decade to 1-2 decades away depending on who you believe. India is set to follow. Many smaller countries are also reversing around now, but those two for obvious reasons have a huge impact.
The last holdouts with high fertility are a handful of sub-Saharan states, with Niger by far the worst (Somalia second - the two are the only countries in the world left with fertility rates above 6), but even Niger is seeing fertility rates plummet as it's economic growth have sped up.
The estimates varies massively based on various scenarios. E.g. on a current trajectory, China will see its population halve by 2100. It's likely you'll see the Chinese government do a lot to prevent the decline from being that steep because it will be an economic nightmare to manage, and so we can't assume the trajectories will remain unchanged.
But nobody is actively trying to get much above-replacement fertility rates, so any political interference is likely to moderate delay the peak not prevent it.
Furthermore, fertility drops as education increases and poverty decreases. This trend is consistent in every population where we can correlate economic development. E.g. here's the fertility rate for Niger [1], one of the poorest countries in the world. Even that has taken a sharp downturn as economic growth has started decoupling from uranium export (which previously dominated Niger's GDP, but provided very little benefit to the population).
Niger, incidentally, has the highest fertility rate in the world, and is one of only 10 remaining countries with fertility rate above 5.
As such such "high fertility subpopulations" are really high poverty, low education populations - currently countries categorised as "Least Developed Countries" by the UN have a fertility rate of 4. Sub-Saharan Africa is the only region of the world that remains above 4, but that too is dropping fast as most of the countries in the region are seeing solid economic growth.
As poverty drops and education increases, fertility rates are dropping across the board, just not at the same rate. But even sub-Saharan African is headed for reversal.
[1] https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location...
It's not really an open question - it would take reversal of long lasting trends in large parts of the world to do more than at most delay the reversal.