I disagree with your position because it is based on the premise that culture/society never evolves. It does, and that is what people are afraid of, whether they are in the Northeast of India or Germany or Nigeria.
The correct response should be, cultures always get enriched by other cultures, and they should not fear "others". The concept of the nation state really added more fuel to the fire (than it needed), but emphasizing nationhood as something that was primary, and therefore its concept (read as culture/language etc), needed to be protected and maintained.
If you were to dig further into the history of the culture in the Northeast, they did evolve over time, perhaps not as rapidly, but they did. And with the advent of information (phone/internet/tv) these cultures will evolve further without even the existence of other cultures within their midst.