Even if global population continued to increase forever, that doesn't mean a particular nation would be able to get those immigrants to come fix their demographic woes. It doesn't matter how easy Moldova makes it for people to immigrate, who is choosing to go there over other destinations just as in need of fresh blood? Countries like the US and western Europe maybe can solve their demographic problems with immigration for the next few decades, that doesn't mean that its a generally viable strategy or that it will keep working in a future world when these nations are well past their prime.