Your prejudice is that Indians would be a net negative to the economy. Even if they were only fit for lower-skilled work, they'd free up the native workers for native-specific jobs.

Societies aren’t just, or even primarily, economies. Immigrants also have an impact on the society and its culture. I once had a conversation with an American-raised Japanese person who had moved back to Tokyo as an adult. He was telling me about the frustration native Japanese had with his lack of knowledge and familiarity with Japanese rules and social customs. These are rules and norms children are socialized into from birth, by parents who experienced the same socialization themselves. People who don’t know the rules create friction in a well ordered society. I haven’t been to South Korea, but I don’t understand it to be materially different in that respect.

As someone from the subcontinent myself, I can pretty much guarantee you that a large influx of Indians wouldn’t be able to follow the rules of South Korea. It would create disorder and disharmony in society.

It would also be bad for the immigrants themselves, because they will never be integrated (South Koreans will never recognize immigrants on equal footing) and will become a permanent underclass. You see this with Turkish immigrants in Germany—and that is an individualistic western society. That’s going to be an even bigger problem in a more collectivist society like South Korea.