In an opinion piece for the New York Times entitled The Right Is Furious With Liberal White Women Michelle Goldberg includes this paragraph which I quote:
The Canadian influencer Lauren Chen … wrote that the ideology of women like Renee Good is “almost wholly responsible for the decline of Western civilization.”
That idea is outrageous on many levels, not least because the gradual takeover of what remains of The West by authoritarian figures who now lead most Western nations is both a product of the “decline of Western Civilization” and a driver for even further decline as defined by the historian Oswald Spengler, whose findings were mentioned in an article entitled Oswald Spengler on Cosmopolis and Depopulation in the Population and Development Review (2004). This is part of the abstract of that paper:
Historians are professionally averse to grand civilizational themes, especially where predictions may be entailed. The German historian Oswald Spengler (1880–1936), … was an exception. His two-volume magnum opus, The Decline of the West, published in 1918 and 1922 (English translation, 1928), … presents an enormously ambitious tableau of universal history seen as the unfolding of the fates of eight cultures, with a focus on four main strands: Indian, Classical, Arabian, and Western. In Spengler's interpretation, imbued with cultural and historical pessimism, the West was exhibiting symptoms found in earlier civilizations in decline. “Civilization,” in Spengler's vision, was a stage that follows cultural flowering — creative manifestations of the culture's unique soul expressed in art and thought. Civilization's preoccupation is with the enjoyment of material comforts; the sequence from “culture” to “civilization” represents the very antithesis of progress. Spengler saw the West as having entered that latter phase in the nineteenth century: .... As longer-term consequences (for the West “between 2000 and 2200”) Spengler foresaw the “formation of Caesarism”; “victory of force-politics over money”; “increasing primitiveness of political forms”; and “inward decline of the nations into a formless population, and constitution thereof as an Imperium of gradually increasing crudity and despotism.” As to demographic consequences, Spengler highlights the emerging “sterility of civilized man “— “an essentially metaphysical turn toward death.” “Children do not happen, not because children have become impossible, but principally because intelligence at the peak of intensity can no longer find any reason for their existence.” “Prudent limitation of the number of births” eventually leads to a “stage, which lasts for centuries, of appalling depopulation.” Immigration apart, the time scale specified by Spengler for depopulation — “for centuries” — may be seen today as relatively cautious. Should Europe's current period fertility level —slightly below a TFR of 1.4 — be translated into cohort performance, it would yield an intrinsic annual rate of population growth of roughly -1.5 percent. Within 200 years, such a growth rate would reduce a population to 5 percent of its original size.
As an initial aside, it is ludicrous to charge a mother of 3 children as being even partly responsible for a reduction in population. Renee Good did her part to contribute to the population of the United States. She exceeded the Total Fertility Rate (TFR) of 2.1, commonly called the “replacement rate.”

But if you delve deep enough into right-wing literature, including the statements and writings of figures like the enormously powerful Stephen Miller, who also has 3 children, his real objection is that not enough white babies are being created versus the non-white immigrants being allowed into the United States who have a higher TFR. This is an issue which also confronts Europe as described in the above abstract. Right-wing political leaders in Europe make the same arguments that Miller does about the need to better control immigrant populations or else the basic nature of Europe will be permanently changed.

Other figures on the right object to women having any role outside of the home, an idea that is also advocated by the Taliban in Afghanistan. Those people advocate for the repeal of the 19th Amendment of the Constitution which gave women the right to vote. That idea is fed from the religious idea that a woman should be subservient to her husband and thus her vote should be cast as her husband desires. To eliminate any possibility of casting a wrong vote, women should lose any right to vote. To counter the ongoing trend of depopulation, abortion and contraception need to be prohibited so more babies are born. Finally, women should return to the earlier state of being essentially the property of their fathers and husbands. This is an idea which is key to the culture of several devout Muslim nations, including, again, the Taliban.

There are only two reasons you might need more people. One would be if we needed more workers after the extra kids grow up a couple of decades into the future. The other would be if we needed a larger military force to defend our land or conquer other lands in that same timeframe. I rather fear that Miller and his compatriots are striving for the latter. Certainly, if we listen to Elon Musk about the prospects of Artificial Intelligence (AI), we aren’t going to need more workers. Musk asserts publicly that most people will live a life of quiet luxury once we turn our economy over to AI-driven robots. Even more reason to stop immigration now before we get too many foreign-born people in the United States.

But the ongoing depopulation of most Western nations (if we exclude immigration from other, non-Western nations) is an effect of the decline of the West. It is not its cause. The cause is “civilization's preoccupation … with the enjoyment of material comforts; …” To understand this we need to look at what Spengler calls the “Culture” era of the West, which is most closely related to the Ancien régime of France.

Western Civilization had its beginning with the crowning of the Frankish king, Charles the Great, or Charlemagne, as Emperor of the Romans by Pope Leo III in Rome on December 25, 800 CE. That act married together the Roman Catholic Church with the Frankish military led by Charlemagne and imbued it with the idea that they were taking forward the old Western Roman Empire which had died in the 5th century. This is why it is called “Western Civilization” because it took on the mantle of the Western Roman Empire.

The Frankish social order then consisted of “those who pray” (clergy), “those who fight” (nobility), and “those who work” (everyone else). Those are the “three estates” of classical society. (It is self-aggrandizement for journalists to refer to themselves as members of the “fourth estate.”) The Holy Roman Empire, the political system founded by the crowning of Charlemagne, formally ended on August 6, 1806, when the last Holy Roman Emperor, Francis II, abdicated on the orders of Napoleon Bonaparte.

That puts a nice single date onto a process which took place over many decades where people mostly threw out the idea of rule by a class (“estate”) of nobles and converted to some other form of democratic rule or authoritarian rule by a non-noble person. The longer process includes the revolutions of the United States against England, the French people against their own king, and later less-dramatic revolutions within other nations of Europe.

In Spengler’s view, democracy and capitalism are a brief intermediate period between rule by a traditional nobility and rule by a purely autocratic non-noble person, such as the series of Roman Emperors who ruled at the end of Classical Civilization. I again quote the above:
Spengler foresaw the “formation of Caesarism”; “victory of force-politics over money”; “increasing primitiveness of political forms”; and “inward decline of the nations into a formless population, and constitution thereof as an Imperium of gradually increasing crudity and despotism.”
We only need to look at the quantity of Russian oligarchs who happen to fall out of windows on high-rise buildings to understand what the “victory of force-politics over money” means. While Trump isn’t executing wealthy people (yet), he is extorting money from them by various means. And that is just the early stage of what Putin accomplished long ago. While Putin maintains a front of having periodic elections (where no real opponent is allowed to run for office), he is effectively an absolute monarch.
"L'état, c'est moi" (pronounced "lay-tah, say mwah") is a French phrase meaning "The state, it is I," famously attributed to King Louis XIV of France, symbolizing absolute monarchy, where the ruler embodies the nation, holding ultimate authority, power, and responsibility, with no separation between the person of the king and the government. It signifies that the monarch is the nation, with all its power residing in their person, a concept central to absolutism.
In his mind, Trump believes he is already at that point where Putin has long been. It remains to be seen if the citizens of the United States will allow Trump to get away with making himself the absolute monarch of what remains of the United States.

But all of this ties together into an example that, while Spengler didn’t yet see the end state of The West in view, he saw “The Decline of the West” in progress, which is why that was the title of his book. The cause of the decline of The West was the rejection of monarchical rule. That and the beginning of allowing wealthy individuals to run things for their own benefit. Average folks have always been left out in the cold. The result is a return to autocratic rule by a series of “populist” authoritarian leaders. “The West” has declined for over two centuries, and what remains for the near future is:
... the “formation of Caesarism”; “victory of force-politics over money”; “increasing primitiveness of political forms”; and “inward decline of the nations into a formless population, and constitution thereof as an Imperium of gradually increasing crudity and despotism.”
Most of us who are deeply steeped in the freedoms called for in the Declaration of Independence reject the idea of “an Imperium of gradually increasing crudity and despotism.” But it would be wrong to try to stem the tied of declining Western Civilization. Its fate was sealed over 1200 years ago when Charlemagne was crowned. If we want freedom for ourselves and our posterity, we will need to invent a new civilization to replace Western Civilization. And that is a tall order for those who have the vision to see the necessity of this, the real replacement theory. We should keep a capitalist economic system while building a strong and impenetrable wall between money and politics. Only if people are truly free of political coercion will they be free to elect the best people to represent them in political situations.