Why not use them, if they're more qualified?
The only reason why employers can fleece immigrants in US today, is because immigrants are effectively "locked in" - e.g. if you're on H1-B, you can't change employers without going through a bunch of paperwork; and if you get fired, they just kick you out of the country. Which, obviously, means that you can't negotiate from the same position of strength as a native. But remove those, and why do you think an immigrant would demand to be paid any less? We're not stupid.
Because you're poor. An Indian in India makes 1/20th the wage a US citizen makes in the US. If they get 1/15th you have just received a huge raise and are happy.
Besides, why would someone take 1/15th, if they can have the whole thing? I mean, by a similar logic, you'd expect people in US readily undercut each other at 1/2th, 1/4th etc in a race to the bottom. But we don't.
Listening to the dumps some of my co workers live in I can see why median hourly wages have not increased in the last 40 years.
This is political economy 101. One of Malthus's suggestions for reducing the mortality rate was getting exotic materials to become staples of life so food wouldn't be the limiting factor of population.
An immigrant will capture a much smaller share of the value they generate, otherwise why use them, than a native born citizen.
This means that while theoretically 'wealth' is increasing it is increasing in such a way that those who are in the position to pay wages keep more of them and those in the position to receive them get much less.
In short then what's the point of an average in a power law distribution which by definition does not have a defined average?
Or to put it even more dumbly would you rather live in a world of 100 units of wealth distributed between the 5 quintiles as [80.0, 16.0, 3.2, 0.64, 0.128] or one of 20 units distributed as [6,5,4,3,2]. For 2/3rds of the people in the world the second would be better.