No country is running a "population must increase forever". You only hear that when the public pensions are discussed because they are unsustainable. The argument is not " population must increase", it's more "human labor is the most critical resource and we must get as much as we can".

You can fear the results of runaway immigration in the short term, like cultural clashes, organized crime and brown people in your neighborhood. But you can't deny the results on the long term when you allow talent to go to your country and end up with more nobel laureates of New Zealand origin than New Zealand.