That would be true if Japan was the only country experiencing this at this moment in time but it's a global phenomenon affecting both good economies and bad ones . Focusing in on the Lost decade of a single country prevents you from seeing that the country right next door was having the best economy in their entire civilization in South Korea and experiencing an even worse birth rate than Japan. same story with Taiwan where the conditions were magnificent
Birth rates in all of Eastern Europe, northern Europe , South Asia, Latin America, etc all fell off the cliff despite their country's experiencing varying levels of economic conditions. for example look at the Singapore economy which is doing fantastically, is doing so well that the government can afford to subsidize everything from housing to food. But look at their birth rate.
It's a bigger than economics. It's a global social phenomenon. You have to see the big picture. It's not the fault of any one corporation or any one country's economic conditions. Countries with horrible conditions like India or brazil are in the exact same position as countries like Singapore or Norway which actively coddle their population with subsidies and wealth.
There is truth in what you say, and no one has the full answer. I used to study demographics years ago (even got a paper out of it) and part of it was demographics of fertility. I didn't research the fertility decline directly but read some opinions of demographers who did.
First, what enables fertility drops is the ease of availability of birth control like condoms and especially the pill (duh). Believe it or not, abortion actually doesn't play a huge role in limiting population fertility like many believe but that is a different, deeper discussion. However, birth control isn't the only thing because there were huge drops of fertility in other times (like the 1920s).
Cost of living, especially housing and the average cost to educate a child are huge parts of it, especially in developed Asia, but if that was all, economic grants and incentives should solve the problem. However, there is another factor.
I want to be clear I am not proposing nor supporting anti-women or regressive ideologies but here is what some demographers think a key underlying cause is:
A lot of the decline in fertility often occurs simultaneously with the increase in educational attainment by women in the population. It's not just that educated women want smaller families, it is the fact that they marry later (and female average age of marriage or first child is tightly locked with population fertility) and have children later which limits the potential fertility for women and also raises the risk of fertility issues which rear their head in having kids post 30 etc. This is also why countries like Japan, India, and parts of Latin America with a traditionally lower female workforce participation still saw fertility declines. The women worked less than other countries but still got educated.
Educated women also don't require marriage to survive. This is not saying they are "I don't need a man" or ultra-feminist. It just means they don't need to get married and eventually have kids as an act of economic survival and social proof.
Now a lot of right wingers run with this obviously. In bashing female education or workforce participation though, what they don't mention is the same demographers also explain that not educating women causes a huge hit in the potential economic growth and overall standard of living in a population. So this every woman is a trad wife fantasy won't work if you want anything near the standard of living and economic dynamism we have today.
Just my two cents.