It was really scary not knowing what the correct thing to think on this issue was so I feel a lot better now that a large corporation has demonstrated the ethically suitable position.
Japan is essentially a monoculture. Other than their below-replacement birth rate, they don't seem to be struggling -- at least not based on any (non-existing) plan to diversify.
The same is true for China.
And India.
In fact, let's just cut to the chase: it is only the west that seems to subscribe to this notion.
P.S. Here's a culture that does appear to be "struggling" with / from diversity: Russia.