We are not in a demographic death spiral. The median age in the US is 38.5, which is 61st in the world. China is 38.4, which is 62nd in the world. India is 28.7, 141st.
The total fertility rate in the US is 1.84. In China, it is 1.6. Do you think China is in a demographic death spiral? China is still growing at .26% a year compared to our .7%. China has 1.4 billion people. We have 332 million.
You don’t understand population momentum. Population still climbs for decades after TFR drops below replacement level