I don't follow? Why would a US citizen want to compete against 8B people for jobs instead of just 300M? Seems like the lower number is better for the citizens of the US.
Immigrants contribute to both supply and demand. It's not at all obvious whether this is net good or bad for natives' pay.
The job should go to the most qualified person. That's how america works. If that most qualified person is actually a guy who was born in india, great. I have no problem with that. You don't get to bogart opportunity based one what zip code you were born in.