Steyn deals with the demographic but consistently misses out on the underlying loss of soul of the western civilization. He sees it, calls it "Post Christian", worries about the "welfare state" aspects, but still stops short of the main cause:
Ducunt Fata volentem, nolentem trahunt!
"...Not merely the importance but the very existence of industry depends upon the existence of the hundred thousand talented, rigorously schooled brains that command the technique and develop it onward and onward. The quiet engineer it is who is the machine's master and destiny. His thought is as possibility what the machine is as actuality. There have been fears, thoroughly materialistic fears, of the exhaustion of the coal-fields. But so long as there are worthy technical path-finders, dangers of this sort have no existence. When, and only when, the crop of recruits for this army fails - this army whose thought-work forms one inward unit with the work of the machine - the industry must flicker out in spite of all that managerial energy and the workers can-do. Suppose that, in future generations, the most gifted minds were to find their soul's health more important than all the powers of this world; suppose that, under the influence of the metaphysic and mysticism that is taking the place of rationalism today, the very elite of intellect that is now concerned with the machine comes to be overpowered by a growing sense of its Satanism (it is the step from Roger Bacon to Bernard of Clairvaux) - then nothing can hinder the end of this great drama that has been a play of intellects, with hands as mere auxiliaries."
In another chapter Oswald Spengler continues:
"...When reason have to be put forward at all in a question of life, life itself has become questionable. At that point begins prudent limitation of the number of births. The primary woman, the peasant woman, is mother. The whole vocation towards which she has yearned from childhood is included in that one word. But now emerges the Ibsen woman, the comrade, the heroine of a whole megalopolitan literature from Northern drama to Parisian novel. Instead of children, she has soul-conflicts; marriage is a craft-art for the achievement of "mutual understanding." It is all the same whether the case against children is the American lady's who would not miss a season for anything, or the Parisienne's who fears that her lover would leave her, or an Ibsen heroine's who "belongs to herself" - they all belong to themselves and they are all unfruitful...
At this level all Civilizations enter upon a stage, which last for centuries, of appalling depopulation. The whole pyramid of cultural man vanishes. It crumbles from the summit, first the world-cities, then the provincial forms and finally the land itself, whose best blood has incontinently poured into the towns, merely to bolster them up awhile. At the last, only the primitive blood remains, alive, but robbed of its strongest and most promising elements...
Consequently we find everywhere in these Civilizations that the provincial cities at an early stage, and the giant cities in turn at the end of the evolution, stand empty, harbouring in their stone masses a small population of fellaheen who shelter in them as the men of the Stone Age sheltered in caves and pile-dwellings. Samarra was abndoned by the tenth century; Pataliputra, Asoka's capital, was an immense and completely uninhabited waste of houses when the Chinese traveller Hsuan Tsang visited it about A.D. 635, and many of the great Maya cities must have been in that condition even in Cortez's time. In a long series of Classical writers from Polybius onward we read of old, renowned cities in which the streets have become lines of empty, crumbling shells, where the cattle browse in forum and gymnasium, and the amphitheatre is a sown field, dotted with emergent statues and hermae. Rome had in the fifth century of our era the population of a village, but its Imperial palaces were still habitable."
Although I have a catholic viewpoint on world-history and identify with the "fellaheen" of Spengler, his words are wise and prophetic beyond belief.