What Happens When a Country's Culture is Replaced by Immigrants?

Could a large, democratic country admit so many immigrants that they would replace the native language, culture and identity? David Coleman, an Oxford demographer, thinks that it could happen. The scale of modern migration, and high immigrant birthrates has led him to speculate about a “third demographic transition” in which immigrants replace the language, culture and ethnicity of receiving countries.
Ethnic change can, of course, happen in the wake of conquest: Han Chinese are rapidly displacing native language and culture of Tibet, where Tibetans have become a disadvantaged minority. But would a democratic nation allow its ethnicity and language to be replaced by those of an immigrant group?

What Happens When a Country's Culture is Replaced by Immigrants?