The Sri Lankan civil war wasn't a Muslim conflict.. I believe it was a classic ethnic conflict between the mainly Buddhist Sinhalese majority and the mainly Hindu Tamil minority, both being long-established populations in Sri Lanka rather than in either case being an immigrant population. Addressing another of your points, the major developed nation with the oldest population, Japan, doesn't permit much immigration and has little apparent interest in changing this.  And China's population is poised to quickly age as well but it likely won't promote immigration as a solution. There's little concrete proof that large-scale immigration is a viable solution to looming demographic change, either here or elsewhere. As new technologies render work increasingly redundant, pursuing a policy of raising the population could simply result in greater economic insecurity and stress.