"Theory"...? It's a published fact. By the UN.

https://www.un.org/development/desa/pd/sites/www.un.org.development.desa.pd/files/unpd-egm_200010_un_2001_replacementmigration.pdf

The attached screenshot comes from page 22 of that PDF from the official UN site.

(Obligatory "But wait, why is he saying this is antisemitic, what does this have to do with Jews?")