It's not just the raw numbers, it's the ratio between old and young that is absolutely essential which you're not including. Say we're below replacement rate a

It's not just the raw numbers, it's the ratio between old and young that is absolutely essential which you're not including. Say we're below replacement rate and the first "halving" has occurred. The replacement rate now no longer is 2.1 (as it currently is), it would be perhaps 3-4. Which won't happen, so then it becomes 5. Which won't happen so then it becomes 6. And so on. Once the serious decline sets in, it cannot be stopped.