If frogs in the well like you were dominant in stone age, we would still be living in caves. Imagine going „we don‘t want anything to do with other cave cultures to form a village“, „we hate those villagists“ etc.
In a hundred years or so when you look back, those little frog in the well cultures would have reduced to nothing (already happening because of demographic problems) and the dominant cultures would be the ones that can absorb new people and grow bigger.
They are importing hundreds of thousands of immigrants from India, Pakistan and recently Africa. If they were to accept a limited number of highly skilled workers from those regions, it would've been fine. But such numbers, and of low-skill workers at that, that is No Bueno.
You probably don't know what these people are like, I do. And let's just say, they are extremely incompatible with Japan. It is not about Religion or such. Most core values are universal: Islam, Christianity and Buddism basically agree on what is good and what is bad. The main problem is culture: an amalgamation of "traditions" and "mindset", a view on how one should live his/her life. That's where a VAAAASST difference exists.
Those people from the aforementionned regions will form communities on their own, especially with their numbers, interbreed and eventually outnumber the Japanese in Japan with their much higher fertility rates. If you thought Europe and US were bad, you'd be in for a nasty had Japan continued on the same trajectory.
In theory, importing south east asians should be fine but, I'll be blunt, everyone knows at this point that no one hates Asians more than their neighboring Asians. This is especially true for Japan, so consider me skeptical.
It seems, to me at least, that the Japanese will have to accept and adapt to a lower standard of living. What they previously attempted to do risked wiping out their civilization over time. Such is the fate of western Europe, unless a giga bloodbath occurs.
The Japanese's natural aversion to foreigners might've saved them, but we'll see. A dramatic drop in living standards might also push them into a neo-sengoku jidai. Old habits die hard after all, we've seen that recently with the assasination of Abe.