Then we get eventually get a population reduction. If you're an economist this is bad. If you're old, this ain't great. If you're interested in preserving at least a little bit of nature and the environment on this planet though, it's likely a good thing.
At some point you need birth rates to stabilize for a population to survive. This is not something that _any_ country with a GDP over $5,000 PPP is currently managing.
The result isn't a population reduction, it is a population extinction. Too few births is just as bad as too many. It's just that you don't notice you've decided to go extinct until 30 years after the point of no return.
The current average population increase is estimated at 81 million people per year.