This strategy works as long as there are more places (by volume) experiencing growth than decline. Since the trend is slower growth overall, there will be a point where global growth stops, and clearly then the strategy will start to fail.
Frankly, from a planet point of view I'd hope that point comes sooner than later.
This will play out in obvious ways (lifting retirement age etc) but ultimately the quality of life will increase overall until some sort of stable population number emerges.
And, if she doesn't do that, then some other little girl in Japan has to say that same thing but with a number higher than 2.1.
Why would they buck the trends of their own ancestors, their own family?
Surely, it looks similar to the ancient past where some lineage looks as if the same happened. But that was because only one offspring survived to adulthood, and then of his or her children only one survived. But they were having many more with tragic results. Those children, for as long as they lived, existed in a world where people were trying to have many.
This is the part where people reply to me as if I were crazy. But children who grow up seeing those adults around them having few children internalize that as normal, and don't seek to have more than that number.
I live in Japan and have Japanese friends. Lots of Japanese I know want to have kids, and wouldn't mind having 3 or more kids. They love kids! And it's still the dream of many women here to get married, have kids, and be a housewife since they know how terrible the careers are in this country.
But 1 or 2 is the limit due to work/time, finances, practicality (housing, vehicles/public transit). The families I know with more kids are the ones who can afford having a housewife or who have multi-generational homes (live-in grandparents).
What I'm trying to say is that I think one major reason people aren't having more kids isn't because they don't want them, but because society isn't built to enable them to.