The problem with a sudden collapse of birthrates in most of the industrialised world is the instability it will cause. A bunch of large countries have seen their birthrates drop from 3+ per female to <2 per female, in some cases as low as 0.9-1.2. At the same countries there are a bunch of countries, typically poorer countries in terms of GDP per capita, that still have fast growing populations. This causes a massive pressure for mass migrations from very different cultures, as well as strong generational imbalances. Ideally most of the world would gravitate towards stable 2.1 replacement rates, or not too far from it.

IMO 1.9 or even 1.7 wouldn't be something to worry too much about (many economists would disagree). But going even below there, and adjusting very fast is worrisome. It guarantees a socioeconomic shock.