Tamils aren't native to Sri Lanka. Their culture, language, history all developed and flourished in what is now Tamil Nadu. Everything of note of the Tamils took place and is found in Tamil Nadu. The idea of a Tamil people did not develop in Sri Lanka but in South India. On the other hand the Sinhalese are native to Sri Lanka; their culture, language, history all developed and flourished in the island. Everything of note of the Sinhalese took place and is found in Sri Lanka. The idea of a Sinhalese people developed entirely in Sri Lanka, and not in India.
Secondly I'm not talking about Jaffna Tamils (who have been in Sri Lanka for centuries) or "Sri Lankan Tamils" here, but Indian Tamils who live mainly in central Sri Lanka.
Yes they were.
You mean 'legally brought' by a colonial power that invaded the island and stole all the land for plantations from the natives and then settled foreigners from another country in that land?
Have a look at all the persecution metered out to the Kashmiris, Muslims, Dalits, Christians, Sikhs etc in India before pointing fingers at others. People who live in glass houses shouldn't throw stones.
India has one of the fastest growing economies in the world does it not? It prides itself on becoming the next world power. It has a population of more than 1.2 billion - what is an extra few million citizens going to do to the demographics? Nothing much.
You can add the whole population of Sri Lanka (21 million) into India's population and it would have basically a diddly squat effect on India's demographics.