Those aren't related. I'm from Romania, which has:
1. A higher population population than Croatia.
2. Higher emigration than Croatia.
3. A lower fertility rate than Croatia.
And yes, the economy is going up despite people becoming old, dying, or just leaving the country. But the reason for that is temporary, it's because Croatia, just like Romania, is still a developing country, you're converting low productivity workers to higher productivity ones.
But how long do you think this can last? What happens when active people are 1/3rd of the population or less and there are no more easy gains to be had?
I just said that things won't look like in Fallout, so why are you even mentioning "apocalyptic hellhole"?
A country doesn't lose 20% or more of its population without major changes happening. It's extremely optimistic to hope that there won't be at least some major negative changes.