People, all of us really, are prone to talk of ideology as a cause; and those of us who recognize the decline of the West are prone to blame it on Leftism (and for Christians, secular Leftism) - but this is only proximately true.
Leftism is indeed the mechanism by which decline is - as it were - implemented; but that is not where the drive for decline comes from - in other words, Leftism is not the ultimate cause, it a means to the end of decline.
So where does the decline originate? There are (at least) two levels of answer - depending on where you locate ultimate causes. The candidates are biology and religion.
For secular people, the ultimate cause must be biology, since biology is the ultimate cause for social phenomena in a secular explanatory model - in other words, the West is declining due to natural selection (since that is the ultimate explanation for everything in biology - or, at least, matters related to change, adaptation etc.).
So what kind of biological change might explain the decline of the West?
The most usual explanation is related to change in selection pressure under conditions of modernity (since the Industrial revolution) - in a nutshell that modern conditions select for lower intelligence, short-termist time horizons, parasitic/ exploitative behaviour and so on.
This would mean that the West was still adaptive, biological fitness (reproductive capability) is still the same (or improving) - we are still getting better at doing stuff; but now the West (and its people) is becoming good at something different from what it used to be good at.
But for me the striking factor of the collapse of the West is not that it rewards the wrong people (wrong from the point of view of sustaining and growing the West) - but that it is just plain maladaptive.
It is not that we solve problems in a different way, but that we don't solve problems.
In particular we don't marry and stay married - we don't even want to marry; we don't have children - we don't even want to have children; we are so utterly terrified of death we have nothing to say about it, nor even for the function of old age.
In other words, it seems to me that the West is broken rather than better at something different - that what we are seeing is an all-round decline in fitness.
The reason for this would probably be mutation accumulation due to the 'relaxation' of natural selection. Humans used to undergo harsh selection at each generation with damaging gene mutations filtered-out; because that only a minority of the 'fittest' children would survive to adulthood, and in adulthood only the 'fittest' minority would provide the majority of the surviving offspring ('fittest' meaning - having the fewest deleterious mutations).
This scenario of decline driven by mutation accumulation would mean that as individuals, and therefore en masse as groups and nations and the world, people fail to understand what is going on and fail to make the right decisions and fail to take the right action: humans have become worse at solving problems. And this will continue to deteriorate.
But, the big question for me is whether this biological explanation suffices to explain the collapse in will - Modern Man is distinguished by his having 'given up'. He does not seem to want to survive, or to reproduce, or to know what is really going on, or really to solve real problems.
For me, a big question is whether this situation can be explained along the lines of Mouse Utopia
in which modern people are so genetically damaged that we, like the mice in Phase D and the Terminal Phase, are simply doomed by our indifference to everything except our own momentary comfort and pleasure and our failure to reproduce - or whether the malaise is primarily spiritual.
As a Christian, my ultimate explanatory model is religious - and related to the destiny of earth; and by this account (and in the opinion of people whose spiritual insight I respect) - we are in the End Times or Latter Days; and that are hope-less maladaptiveness is because we are trying (and failing) to live without God.
(The duration of the End Times is not known - the Bible states it cannot be known (not even Jesus Christ knows this) - and there is always the possibility of human choice and action delaying the End - and even effecting a temporary and/ or local reversal. Nonetheless, once the final phase has started, as it has, then it will sooner or later run to completion.)
So, I tend to think that even if the worst predictions of Mouse Utopia are true, and this is what we are living through - IF we were spiritually healthy, if we (as a society, and as individuals) turned back to God, then we would not be in such a bad state - because we would be pointing in the right direction.
We would not just have given up in a profound existential despair, but we would still be trying and striving, still doing our best, still learning from experience. And, no doubt, much could be done - problems could be ameliorated rather than as (at present) exacerbated; we could, for example, do nothing instead of always making things worse.
So, I would regard the spiritual, 'supernatural', religious explanation for the decline of the West as the deepest and truest - although I think it is very probably interacting with biological change.