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.