I don't think this is true. Every single country is on the decline. They may sill grow in this century, but it's very likely that it would stop in 50 years.
And in the case of Niger, Mali, Nigeria etc, when their TFR would eventually cross the 2.1 limit of demographic transition according to the standard model nearly half the global population would live there at the end of this century. It is insane and completely unsustainable even when not taking climate change and ressources depletion into account
Do you have any data for that? It's supposed to follow development and modernization, which seems to still be lagging.
https://www.newsecuritybeat.org/2013/08/demographic-transiti...
Tl;dr Demographers have previously assumed that since Asia followed the same path, the European model of demographic transition was a universal truth. However they underestimated the impact of cultures (in Africa having large families is considered has an assurance as if one of the children succeed the parents will be protected), religion (in Islam even if contraception is legal, one need a valid reason and procreation is heavily encouraged), traditions, genetic differences (I think I read somewhere the age of puberty is a bit lower for people of West African descent) etc.
A documentary about this phenomenon in Niger: https://youtu.be/btMUecYNBc4
Theres a lot of talk about traditions and cultural norms. Do these people realize that the traditions and culture norms in the US, European, and Asia are not only diverse but have also changed over time as well?
https://www.sciencedirect.com/science/article/pii/S016028960...
"There were strong negative correlations found between national IQ and three national indicators of fertility."
There may be two P-Bombs: America, East Asia and Europe which spirals toward demographic collapse on one hand and Africa and - a bit less - the Islamic World which spirals toward overpopulation on the other hand.
E.g. Egypt which is already overpopulated fails to succeed to drive its fertility rate under 3. Maghreb countries such as Algeria and Morocco actually saw their fertility rates increase again after decreasing for the past decades. And that does even include demographic nightmares like Niger where half the population is systematically under 15