Scotland is facing a demographic crisis in the middle future due to falling birthrates; without immigration, we won't have enough young people to keep the country running. This situation is, as I understand it, pretty representative of the rest of Western Europe.
Demographic crisis is one argument for immigration I find somewhat reasonable. But, I still think it is often overstated. For one thing, real economic performance should always be measured in GDP **per capita**, not mere GDP, so in theory it is independent of absolute population size. So there will be no economic collapse due to demographic contraction at all. There will be pressure on pension systems and similar problems, but nothing catastrophic. It is just not a huge issue. Second, for immigration to be effective in remedying this problem, the immigrants must be economically productive, not a burden. This is not a given with muslim immigrants, especially if they dont integrate well.
Taken together, Id rather go the way of Japan and heavily invest in technology and education, rather than in dubious mass migration schemes.