> Why?
Because the fertility that they see expressed around them by adults acts as the ceiling, not the floor for their own future fertility. If grandpa had 3 and mom had 3, you might only have 1 or 2, you'll very rarely ever have 4 of your own.
This is just the nature of those numbers... a person with 2 kids is, theoretically, working on a third. Until they get too old, and then it's an "oops".
So, it ratchets downward. When someone waits too long and has fewer, then they become the normal that children around them see, those children internalize the new number (or at least some sort of fuzzy average of those adults around them).
One would expect that to decline ever faster. And that's in fact what we see so far... instead of a fertility rate of 1.4 that just holds steady for centuries until extinction, it drops year to year and decade to decade, and will take far less than a millennium.
> Could someone just 60 years ago easily predicted the current situation?
Probably, if they somehow could have shed their 1960s biases. The trend had already started.
> Turns out the Japanese society was one of the most malleable ones that ever existed.
It's not malleable at all. Adaptable is one thing. Adaptation is an active verb, each person individually and all collectively will themselves to change, so to speak, to overcome challenges. They do that occasionally.
Malleability suggests that some outside force can come along and change them against their will, or at least without them noticing... and they are less malleable than many other cultures. If they were malleable, then we'd expect far more results from the various programs that the government puts in place to alter their culture. And those programs have fallen flat on their faces. They don't make a dent in Japanese work culture, they won't make a dent in Japanese fertility.
We're probably only 25 years or less away from 0.3 fertility rate. By about that time, it won't be so easy to argue against those who say what I'm saying... so those who did will have moved on to "this is a good thing". Some are already hinting it in the other replies. They hate humans, they wish they never existed, and they are secretly cheering this on. (Yet, I'm the fascist somehow.)
This would make me sad, except that it's difficult to be too upset about people who wish they never existed making themselves extinct.