It makes everything too static, and it's not particularly supported by lore. The history of the world in ASoIaF is full of peoples who drove other peoples out of their lands and brought their faiths with them: the First Men did this to the CotF, the Andals did this to the First Men, the Rhoynar did this to the Andals in Dorne, the Valyrians did this to pretty much everyone, and in the time of the books the R'hllorites are doing this across the world.
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