I think people underestimate how many people move back to their home country once they have a better chance (through e.g. education or money) and / or when the situation there improves (e.g. stability). It's why I don't understand why the anti-immigration parties don't do more internationally to help other countries.
Because we, correctly, assume that some countries are simply beyond saving. Throwing good money after bad.
The reason why pouring money into countries that source immigrants is a questionable solution is graft.