There is no great mystery to the demographic decline of Europeans. It is caused by some of Secular Europe's more ‘progressive’ and ‘enlightened’ post-Christian dogmas.
These include the ‘common sense’ of contraception; the ‘right’ to abortion, the ‘equality’ of homosexual relationships. Women - thanks to contraception, giving their most fertile years to a career - not to mention to a live in lover in no hurry to commit. The ‘normality’of promiscuity leading to the devastating consequences of AIDS in some countries and infertile young women in others. (Also, of course, to the sin and sorrow of adultery and divorce.)
These dogmas combine to produce, over time, an exponential decline in births. This results not only in economic collapse with shrinking markets and tax base, deflation, crumbling welfare systems, etc), but also in eventual extinction. Of course, nobody acknowledges the causes of the falling birth rate and its devastating consequences. This after all, would be to admit to the absurdity of ‘enlightened’ attitudes in the face of harsh reality.
All in all, it seems that by dismissing the divinity of Christ and by rejecting its roots in Catholicism, Western Civilisation has created a ‘nice’ make-believe world which is, ultimately, unsustainable. Who would have believed the Catholic Church would outlive the secular triumph? Which brings to mind Christ’s words on establishing it - that ‘the gates of Hell will never overcome it.’ (Matthew 16:18). So much for ‘survival of the fittest’ then. The Catholic Church has been vindicated!