Wouldn’t they benefit from more people? Of course it would come with the condition of learning the language at an acceptable level and that being tied to residency.
The problem is that birthdate is dropping even faster in those countries. An even bigger “problem” is that in general life is getting better, even in developing countries. There is no infinite supply of immigrants waiting to save the developed world.
Encouraging immigration is far from a panacea. It will work for a few countries, for another generation or so, but you can see the end of that coming