Other first world countries. The global population overall is still slightly increasing because 3rd world countries have plenty of children.
As for the why: because historically that has always been a solution to lacking specialized talent or simply not having enough hands to perform a massive task. You don't want to pay domestic labor, so bring in foreign labor. This time it's a general problem.
In your solution you also wouldn't have to kill people off. They would die naturally due to a lack of healthcare.