There was a fic from Lucius Malfoy's perspective where something similar happened. Harry negotiated an agreement where Voldemort and his Death Eathers got England, but were forbidden from interacting with muggles and had to allow everyone who didn't want to live under his rule to leave (including new muggleborn). Voldemort's society rapidly imploded due to inbreeding and the fact that the hardcore purebloods who remained didn't know how to run an economy when the lower class that actually did all the real work left.
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"Anyone can be a winner if their definition of victory is flexible enough." - The DM of the Rings XXXV
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"Anyone can be a winner if their definition of victory is flexible enough." - The DM of the Rings XXXV