Debunk This: The Great Replacement

While writting this down I wasn't sure if I should include numbers and data, or merely explain the concept. I decided on the later because I thought this OP was getting large enough and we could crunch and analyze data and studies later.

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The Great Replacement is a far-right political theory (can it even be called theory? it's really just more of a concept) according to which a native population in a given territory is being replaced on a large-scale by a foreign population from another territory, over the course of a few generations (2-3).

While this theory encapsulate most of the xenophobic fears and group-identity concerns found in right-wing political circles all over the western world, it is mostly debated under the name "Great Replacement" in France. But as a concept it is something that would definitely be recognized in the USA, Canada, Germany, etc.

While this theory branches out into wild conspiracies about european genocide (the Kalergi Plan, which is crazy and doesn't need further debunking and was discussed here at least once) and good old fashioned elite/jewish/shadowy cabal involvement, I would like it if this reddit would take the time to discuss and maybe debunk the base core of this theory.

The ontological basis for the Great Replacement are observable demographic changes and movements: population decline in one place, demographic growth in another place, and an influx of population between the two. This is what, I think, needs debunking.

Is this a real phenomenon? Is the context of irreversible population decline in some european countries, coupled with massive/uncontrolled migration movement from the south, resulting in a trend that will have for outcome the replacement of the native population?

In my opinion, I will say that demographic projections are not reliable beyond a very short point (4-5 years?) because they are dependent on changing contexts (war and stability for instance, economics), and so we cannot base long-term or even mid-term trends on them.

Another thing is that while there's definitely some truth to the concept of Great Replacement (there's is something like a demographic decline in Europe (Link to UN projection and propositions) and there is a population boom in Africa and there is an influx of migrants that isn't managed as it should be and therefore feels inflicted by the native population), the meaning we ascribe to this reality is another debate entirely. And there is an on-going political fight for the meaning we will collectively ascribe to demographic changes in the West, and looking at Trump and at the decline of social-democratic parties in Europe (those are symptoms and consequences of anxiety), it doesn't look good. This is why I think it needs analysis.