On the population shrinking...

I think you've hit the nail on the head.

The replacement rate to maintain population is, I believe, 2.1 children per couple.

Looking at my family on my mother's side, she had 4 other brothers and sisters, so you have:

5 children in generation 1.

There are currently 5 children in generation 2.

And only 6 children in generation 3.

So far in generation 4 there are only 4 children and it's not likely to get anywhere near replacement level.

Given the ideal replacement rate, there should be:

2 in generation 1

4 children in generation 2

8 children in generation 3

16 children in generation 4

Now, my spouse is 1 of 6 children.

Generation 2 has only 4 children.

Generation 3 has only 5 children.

Both initially large families have shrunk to less than replacement.