Personally I think JP is not that able to 'adopt' foreign concepts. I mean even within the immigrants, there is a racial hierarchy, because let's face it JP is racists; the top being the Nikkei (japanese who have lived and grown up somewhere other than JP, most notably Brazil), then comes people like chinese/korean, 'european' or westerners (basically people that don't look asian but most JP people view westerners as those with blond hair, blue eyes, long pointy noses, etc), and then asians from southeast asia, etc.
So even within this hierarchy, they are all still considered 外人 (foreigners). The concept of having foreigners having babies with japanese in Japan just seems too much of a different way of thinking for JP politicians, realistically speaking. That is what I think so far. The question of immigration solving JP's aging population seems like it would take a really long time for that to actually be accepted socially and as part of their culture. . .