For a part maybe, but some of the proponents of mass migration truly analyse the situation only through the economic lens, as for them a civilisation is reducible to its economy.

They also seem to lack the basic mathematical notion that one doesn’t add things that are different in nature: “on additionne pas des choux et des carottes”. For them a population size is a population size with no thought given to the underlying composition of the population.

Which is the main reason all their mental models on this particular topic are often so wrong in practice

What a bizzare analogy and false equivalence. Where indigenous populations are declining, it's mostly due to the consequences of the global economic system that ethnic Europeans participate in, as well as outright persecution and colonialism. We already are replacing them.

"Don't feed egregious comments by replying; flag them instead."

a.k.a. please don't feed the trolls

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