I feel you on SK and demographics, but 3 generations is still a very long time these days. We're talking close to 200 years before anything like a population collapse.
Only if you count as "people dying and then the next". After a parcel of a population reaches adulthood, another generation is already expected. See the delta between GenX, Millenials, GenZ, and so on. I'd say that scenario is closer to 40 years.
The "coming sooner" problem with such low fertility rate is the bell-curve of your population by age. It starts to center on lower-productivity ages, more medical resources are needed. The system can't sustain itself with a thinner base on age.
