>You can only immigrate a finite number of people.

The same like with any imports. Market responds to demand. China has 50M university students, US - 20M. That means that upon achieving US percentage (in probably 5-10 years) China can be having 80M students - sufficient enough to have some of that satisfy US demand for college graduates.

> People in those countries also are below replacement like India which is unprecedented.

Quick google shows that population growth in India is considered a problem, and so they actively trying to decrease birth rate.

>This is nothing to say of the social backlash mass immigration seems to be having across many countries.

Any technological (and mass migration is a result of technological progress) shifts and its consequences cause social stress. Successful societies adapt.