Yea, NO.
You are wrong.
They have an upside down birth rate and a population the will be unable to sustain its’ present level of output regardless of automation.
Four grandparents with for every grand child is not sustainable in a welfare state. No matter how many robots you have.
They will stop being 100% Japanese when they are invaded and subjugated by a more prolific neighbor.
That is the nature of man and the way of the World.
Japan doesn’t need low-wage immigrants to keep up its rate of production. All it needs to do is continue to locate branch plants, wherever those low-wage workers already live. Japan will have no problems keeping productivity high.
Japan has been a “head-office economy” for decades now. That means a high-paid, highly skilled, and highly-educated workforce. A high-paid workforce can support several times more retirees than a low-wage workforce.
Low-wage immigrants need far more heavily-subsidized social services than a highly-paid, head-office workforce requires. Factor in the myriad social problems that come with a large, unassimilated population of immigrants; and Japan it’s obvious that Japan has nothing to gain from an open-borders policy.
When Japan’s population stabilizes, it will still have at least as many people it did during the height of their Empire. As a high-tech, high-wage economy, Japan can afford (and operate) a high-tech military. That’s a tremendous force multiplier. Just ask Israel.