Demographics is much like economics in that everyone thinks they understand it and almost no one does. (Schools of economics are again/still teaching Keynesian clap trap which is little better than if the field of political science was teaching that Stalinism and National Socialism were the most effective sort of political system.) A few people do actual research while everyone else engages in wish-fulfillment. And I don’t necessarily mean describing what they wish were the case but wishing that what they are studying supported their world view–pessimistic or optimistic. La Raca in North America and the Muslim Brotherhood both share a gleeful view of masses of immigrants flooding north to completely transform the society they encounter there. Many in Europe and North America and share their view of where demographic trends are headed but with pessimism about it’s results. So all demographic information is evaluated, interpreted, accepted or rejected based on how well it conforms to the narrative that they have accepted as common sense.
Demographic transitions to lower fertility rates are seen as proof of Western decadence, failures of capitalism or the evils of secularism. The observation that changes in total fertility rates are artificially magnified by demographic conditions like rising ages for marriage and child bearing is not well received by either pessimistic northerners or optimistic southern ideologues (or northern ones in the case of India).
Assimilation is a forbidden concept for all. Pessimists don’t believe it can happen; optimistic imperialists see it as a racist counter offensive that can easily be stopped via piety and purity of ideology. Somehow they will convince other immigrants to remain as supposedly pure of ideology as they are even in the face of many benefits to assimilation. History shows that unless an incoming wave of migrants quickly establishes military control over the host society, it is impossible for them to assimilate that culture and will themselves be assimilated, even if they arrive in huge numbers. They may contribute to the culture they join but they won’t supplant it.
The fact that the supposedly threatening societies from the south are undergoing the same demographic transition as the north has, and faster, or that immigration is only high when there are jobs to attract immigrants (Mexican immigration, both legal and illegal is now at about net zero or lower), have little space in the discussion of demographic trends. But then the discussion of demographics rarely moves beyond the phrase “if this trend continues…”. Pity, that.