I'm not sure anyone knows. The total absolute working-age population will still likely be higher in 2060 than it was in 1950, and complemented by a great many productivity-enhancing machines. It's just that the number of elderly will be much larger, and the number of young children much smaller. But that's also a trend that could reverse; the absolute number of children in Japan will still be higher at that point than it was during, say, the Meiji era. Perhaps large families will come back into fashion for whatever reason.
The number of young people is thinning out AND the fertility rate remains low, and these have a multiplicative effect. Compounded, they are an oncoming freight train. There are very real, very serious reasons that the fertility rate is low. For a vast majority of the population, the decision to have children and provide for them is a decision to sentence yourself to a very hard life. Society is set up this way and most people see no issue. I think that suggesting large families "may come back for whatever reason" completely ignores this reality. You reap what you sow.
This is true, but the future is hard to see and there are multiple possible roads. It's not guaranteed to be an extrapolation of the present.
One possibility is a pivot towards immigration, but I think equally likely would be a pivot towards ever-larger financial incentives for parenthood, the kind that would make being a stay-at-home parent start to make financial sense.
A lot of young people put off parenthood not by preference, or because they're so passionate about being a corporate tax accountant, but just because they don't feel they have a choice; only one route offers financial security. That's been the reality in many developed countries since perhaps the 1970s-80s, but it won't necessarily remain true in the years ahead.
This would require that elderly people (who are an ever-growing voting bloc) support this shift of government support towards young people. It might not happen without some kind of external stimulus. But elderly people also tend not to be pro-immigration either, and presumably also don't love the idea of empty schools and playgrounds and dying towns. It's hard to say what will happen.
I agree with everything about what you said. I agree with you that the elderly are unlikely to support young people, unlikely to support immigration, and probably don't like empty schools and dying towns. What I guess they will do is: absolutely nothing. Hold on to power and live out the rest of their days. And that scenario is an extrapolation of the present.
What has changed is that the fertility rate dropped dramatically below the replacement rate. Do you understand the consequences of this?