France has a similar problem.
At the same time the native French are de-populating themselves (with the average age of the native French increasing), the Islamics are breeding without restraint.
Eventually - if the native French don't just surrender first (and I'm not discounting that) - France will face a civil war. A shooting war, requiring soldiers on both sides. A "fight to the finish", after which France will be reconstituted as an Islamic nation, or after which Islamics will be expelled (or worse) from France.
On the Muslim side, there will be no shortage of young males to take up the cause of Allah.
But France may not be able to mount an army of native French large enough to stand against the Muslims. Even if they draft nearly every able-bodied Frenchman. There won't be a large enough cohort from which to draw....
Demography is destiny. Ultimately, morals and culture may not have all that much to do with things when confronted with explosive population growth. Those in the greatest numbers, prevail. Simple, awful - but true.
- John
That's pretty much the point I was making. In the case of Japan, maybe Islam won't be the problem. I'd think the Chinese are more likely to decide they could swallow a decimated, geriatric Japan in one gulp. Plenty of old grudges there, too.
Another possibility is that, if the population of ethnic-Japanese workers gets too small, they'll decided they could bring a few other Asians in to help support the elderly ... and there are plenty of Asian Moslems.