Nobody, not even the Japanese government expects that. They are anticipating a difficulty maintaining their culture against mixing with the world. Which has actually been going on for 70 years now, and continues at a breakneck pace.
Japanese people will come to include all sorts of people, who will happily live together and move forward. Without our permission or control, as folks all over the world do.
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.
White people already do include all sorts of people -- whiteness is, obviously, not a single unified ethnicity, national, cultural, linguistic, or historical background but an aesthetic conception of someone's skin color. Because ethnostatism is stupid and intellectually inconsistent.