The US has its own problem with potential cultural transformation, as Victor Davis Hanson warns, joined by Samuel P. Huntington, with whom I once studied "government," Columbia University's appropriately named version of "political science," each argues in his own way.
We seem unwilling to grasp the immigration nettle, which would require (a) fortifying and militarizing the frontier, (b) actually incarcerating unauthorized border-crossers until they are deported, (c) deporting thousands of people from communities away from the frontier; and (d) doing menial work with our own hands or those of our legally present neighbors.
Neither the "multiculturalist" left nor the corporate welfare right is likely to grasp this nettle. We can only hope that acculturation occurs fast enough to avoid the Mexicanization of large parts of the country and the resultant Belgian/Quêbequois conflicts.
Japan, of course, has its own problems. Population growth has fallen below replacement levels, and the culture is very resistant to immigrants, even to the absorption of Koreans who speak Japanese natively and are in the third and fourth generations.
The absence of immigration contributes to a different set of problems.
"Multiply and replenish the earth" is the first commandment in the Bible, and carries with it implications about family and sexual life that an increasingly secular industrialized world ignores. A Christianity of lesbian priestesses and sexual latitudinarianism contributes unwittingly to its own demise, in the industrial world at least.
Perhaps "revival," originating in Africa or elsewhere in the Third World, is still in prospect, and will upset all mechanistic projections.