The most populous nation on Earth is by Western standards non-religious -- China. I do not mean that everyone is an atheist as demanded by communism. Rather, the folk "religions" that are most common (there is a large portion of the population, mostly educated urbanites, who are not believers in these either) say very little in the way of the issues you mentioned. They have more to do with granting good luck and curing disease.
This has not hurt the population yet, but the one child policy will certainly wreak havoc the country if it is maintained for too long.
When it will be removed (it will have to), I wonder which group will take the most advantage of it and grow faster - the "non religious by western standard" or the various ethnic groups - say in the XianJiang.
Also, the fact that an authoritarian communist country where atheism is promoted still couldn't remove the religious allegiances is interesting. The former USSR tried the same - and failed.
They are therefore already taking advantage of it, but I wonder when it will be removed it the growth differential will equalize.
My guess is that it won't, and the religious represent a very significant percentage of the population - until they become the majority.
Without willing to insult anyone, I wonder if a human group growth with religion, they reach decadence with atheism - then stats again, eventually with a different religion, in a loop.
EDIT: the point is not about "cultural group" but religion. Are these minorities becoming atheist, or do they keep some religious self identification? I guess they still identify as such, even if they don't practice. Culture is hard to bring back, but religion can grow back very quickly
Equating atheism to decadence is pretty funny when you're familiar with, say, the history of the Catholic Church, almost since its inception, or the history of the religious classes of the Muslim world.
Actually, even without the restrictions of the One Child Policy, the ethnic minorities in China are disappearing as distinct cultural groups. Homogenization of culture is a powerful force.
Ironically for your point, this issue seems to be most severely affecting the Uyghurs and the Tibetans, arguably the most religious minorities in China.
Obviously this has not hurt their population.