What countries do you think this is true of? Even, say, France is still growing. Let alone the US.

US birth rate: 1.70 France birth rate: 1.87 Japan birth rate: 1.36 Pakistan birth rate: 3.45

So Pakistan, unlike many more developed countries, has a birth rate higher than the replacement rate, but significantly lower than it’s 1950 to 2000 highs over 6 - and trending downwards.

The population would be expected to drop precipitously, but not soon.

I think Mexico's fertility rate was around 6 in 1975. And is now 2.10 or something. Basically Pakistan is 20 years behind Mexico demographically. Mexico will probably peak in 30 years.

Not sure about Pakistan because a Pakistan of 500 million doesn't seem sustainable.

Your logic is not sound.

As long as birth rate remains above the replacement rate, population will not drop.

Pakistan's birth rate has fallen significantly (but still above the replacement rate), so you will expect population growth to plummet, but not the population number itself

My point is that the current trajectory would soon make the birth rate below the replacement rate.

Are those countries growing with birth rates alone? I thought immigration was what was helping the populations stay stable

The population in most western countries are rising because of births + immigration. If just looking at births alone, then most of these countries would see declining populations.