You are correct, the birth rate is declining, though the global population is still expected to rise until the end of the century before starting a decline.

The problem fundamentally is like said, technological development makes living much more complex and expensive, this inevitably puts people off having children. It doesn't matter where you live, if the society you live in has reached developed status, the same problem will occur. This is inevitably why the trend is downwards because more nations are reaching this status.

It's a paradox of technological advancement, the more technologically advanced we become, and the more rapid the continued advancement, the more effort people need to put in to keep up and the more costly it is to keep up. This translates into higher stress levels and a greater likelihood of people falling behind which inevitably leads to poverty.

If you add that inevitably the more complex society becomes, the better educated people need to be to function in that society due to the technology, inevitably, this leads to children needing to have more and more time and resources put into them to prepare them for adulthood and surviving in society.

This is why fundamentally we need to find a way to break the trend between technological advancement equalling in a more complex society, technology should be making life easier yet the constant change in technology seems to be making it more complex-and thus expensive.

In reality we need to find a way to create stability which is difficult when change typically creates instability, and yet technological advancement creates constant change. So really we need to find a way to create stability in a world of constant technological advancement.

If we could do that, we would likely break the trend between technological advancement constantly leading to increases in the cost of living and raising children.