Actually no, because these immigrants are not going to replace the entire Japanese population in one click, rather they will keep the country running and prevent the economic system from crashing in front of new Japanese kids.
Basically supplement the needed kids to reach/exceed the replacement rate.
I doubt that's going to be the majority of kids in the case of Japan.
Same thing, just gradual instead of instant.
As I said the goal here is to supplement native kids with immigration, not replace them.
Kids are way way easier to integrate than adults, so for immigration, they are probably the best subject.
This applies to Japan too, the difference is they don't accept reality.
America is really an immigrants country, and its easy to integrate there compared to countries which have less of a globalized culture, such arguably Germany and maybe the UK.
Anyone who thinks that it takes ‘magic’ to turn someone who was born in Germany or Japan into a German or Japanese person is implicitly subscribing to some bogus theory of racially pure nations. (See e.g. your sibling comment, by someone who’s persistently been repeating far right talking points over the past few months.)
I agree, there will be difficulties for sure, but immigration is at its easiest for children.
In contrast, Japan represents a small number of peoples, and it's laughable to suggest that someone can become Japanese or German by a mere act of bureaucracy.