> Nobody, not even the Japanese government expects that.
Are you sure?
https://www.bbc.com/news/world-asia-64373950
> They are anticipating a difficulty maintaining their culture against mixing with the world.
Of course. If you don't have children, then you need immigration from countries that do. That will obviously change your culture.
A nation is made up of people of a shared origin, history, culture, ethnicity, language. If you replace the people with immigrants and they create a new culture, the nation is no more.
> Japanese people will come to include all sorts of people, who will happily live together and move forward.
That's like saying white people will come to include all sorts of people.