If the vast majority of people who were born and live in a region want to be an independent country, that's their call. That could be Scotland, Falklands, Catalonia, Northern Cyprus, Texas, Ceuta, Crimea, Tibet, Hong Kong, Quebec, Reunion, etc. If they want to be part of another country then it's still their call, but obviously the other country also has to agree.
(I accept there's probably some discussion about how recent immigrants count, and the size or duration of the majority to make something change but the principal of self determination is one of the most basic rights people have)