There are a couple of problems with this argument.

The most important problem is that many people do not have documentation, but have lived in India for a very long time—since birth, in some cases—and think of themselves and live their lives as Indians.

More specifically:

First, India arguably also has a history of ethnic cleansing and more broadly ethnoreligiously motivated violence, a history that is all too often suppressed.

From the Sunderlal Report: “We can say at a very conservative estimate that in the whole state at least 27 thousand to 40 thousand people lost their lives during and after the police action. …Duty also compels us to add that we had absolutely unimpeachable evidence to the effect that there were instances in which men belonging to the Indian Army and also to the local police took part in looting and even other crimes. During our tour we ga thered, at not a few places, that soldiers encouraged, persuaded and in a few cases even compelled the Hindu mob to loot Muslim shops and houses.”

Similarly, the Gujarat riots suggest state complicity. It is often suggested that Muslims caused the violence. The truth is that the violence was initially by Hindus, on the train, that some Muslims then unjustifiably burnt the train, and then that the state compounded the tragedy by allowing Hindu mobs to rampage across the state, if not actively encouraging them to do so.¹

(To be clear I do not endorse the silence of some parts of the Indian left and liberal élite concerning Pakistan and Bangladesh; it is as abhorrent to me as silence concerning the violence of the Sangh Parivar. And I should probably say that if they had to be compared the Sangh Parivar is better than the Pakistani army.)

Second, it’s not obvious where the “cost and risk” is. No evidence has been produced to suggest that those left out of the NRC commit more crime or cost the state more. Indeed, the implementation of the NRC itself costs the state rather a lot, and threatens to encourage sedition in the northeast.

Third, all parties in India profess to reject two-nation theory. The RSS itself claims that Muslims are also children of Mother India.² And of course Congress claim to support a secular India. Since there is political consensus on this matter of course the idea that loyalty to Bharat or India or whatever conception thereof one supports should be determined independently of religion.

Fourth, many European states indeed have welcomed large numbers of Muslim migrants, often to much greater cost than what India has incurred—for example, Germany and Sweden. These actions need not have been unsustainable had all Europe helped; similarly, migration to India as a whole is not particularly unsustainable because India is very large.

0. https://frontline.thehindu.com/static/html/fl1805/18051140.h...

1. Christophe Jaffrelot, “Communal Riots in Gujarat: The State at Risk?” Working Paper, Heidelberg Papers in South Asian and Comparative Politics (Heidelberg: South Asia Institute, Department of Political Science, University of Heidelberg, July 2003), http://archiv.ub.uni-heidelberg.de/volltextserver/4127/1/hps....

2. https://www.hindustantimes.com/india/the-saffron-muslim/stor...