Has the Japanese work ethic gotten more intense since 1950, when their birth rate was well over three children per woman?

I suspect the proximate cause of the drop is the hysteria about overpopulation that gripped all Asian countries in the 1960-1990s. The governments there raised two generations of people socialized to believe that having too many kids was bad for the country.

Japan and Korea never had anything as barbaric as China’s one-child policy, but the government heavily propagandized population control during the second half of the 20th century. They imposed taxes on families with more than two kids, heavily promoted abortion and sterilization, etc.

So does America.

We've disguised it with massive immigration.

Every developed country has a dismal birth rate. Birth rate and level of development are closely related in pretty much every country, and the causes are probably broadly similar. Last I checked Germany was worse than Japan, yet nobody is saying it's because Germans are working themselves to death.

This is the current culture, but culture can change.

There may come a time when a new way of thinking will spread, causing this trend to reverse.

Japan has the highest fertility rate in asia