I believe there was an United Nations (or maybe International Monetary Fund, can't recall) document where they were worried about developed countries' birth declines. In order to sustain a certain economic output you'd have to maintain a certain rate of births, given that such rate has been declining over the years (go look up the stats on "birth rates") they needed to come up with a solution. This is most likely why there was such emphasis on fixing race relations (in past decades and even now), etc; because they would eventually had to start working on their immigration plans. And since most underdeveloped countries birth rates are way beyond developed ones - it was just a matter of adding one plus one. Another compounding issue to this problem are the death rates. Given that overall health has improved people live longer, but this invariably eats into pension funds. It means that there's not enough people providing economic output for those who've retired. White Replacement Theory is way more complex than "oMg hE's a loW-LiFE raCiSt!1!" comments.

White replacement theory has some grounding, due to high immigration rates and the higher birthrates of ethnic minorities.

No.