That is happening. The replacement rate, if immigration is excluded, is below the 2.1 kids per woman in more and more countries as they develop.

https://worldpopulationreview.com/country-rankings/total-fertility-rate

Humans slow having babies as infant mortality drops. There is lag, causing a boom, but on the other side is a slow decline.

Hans Rosling did lots of talks on it.

Population growth is in fact slowing down and have been doing so for quite some time. But we'll eventually run into the problem where there won't be enough working age people to take care of the elders.

Many countries (nearly all of the developed countries) are having too few kids to maintain their population and it can only be increased/maintained through immigration. Most experts believe that we will top out at around 10.5 billion in 2100 and then there will be a decline.

People are having less kids, it's just that older people also live longer these days, which means it takes longer for the population to decline, still, is happening in a lot of countries, for example, Japan lost 200,000 inhabitants last year due to low fertility rates

We already are. It's the immigration policy that keeps housing demand up.