It’s just a conspiracy theory.
Replacement Migration:
Is it A Solution to Declining and Ageing Populations?
https://www.un.org/development/desa/pd/sites/www.un.org.development.desa.pd/files/unpd-egm_200010_un_2001_replacementmigration.pdf
It is an “option” if you don’t encourage reproduction or you poison the bulk of the reproductive stock into sterility. But 0 of the imports have the mental capacity to run or make things that maintain technological civilization…