Spengler responding to an improper suggestion...
I was certainly stimulated by Oswald Spengler's 'Decline of the West (1918) when I read it; and found the book to be rich in insights. It is indeed seminal.
But I would have to say that I regard is as essentially wrong.
The wrongness is of two types: the use of biological growth and development as the primary metaphor for cultural change; and the model of civilisational change being cyclical.
The biological metaphor is essentially wrong because it leaves-out the religious element - which I regard as primary among cultural drivers (nothing is more powerful for human cultures than religion, nothing more lethal than the lack of religion).
Once religion has been omitted, then biology - in the form, mainly, of sex and sexuality - does indeed become primary; but only in the context of a terminally doomed situation.
The cyclical model is essentially wrong because human history is primarily linear - and in its most vital aspects never repeats. So the most vital thing to consider is the linearity; and it is only against this unidirectional-unfolding that small cycles may be observed.
Christianity recognises this directionality and purposiveness to human history - and all Christians really ought to have this as a built-in metaphysical assumption - Time is linear, sequential, and without exact repetition.
(However, many Christians have-been and are confused by their metaphysical assumptions that God is outside Time as well as Space. But at any rate, the sequential linearity of Time certainly applies to mortal life on earth - bounded at each end by the first and last Men; by creation and the end of things.)
In sum - Spengler is certainly worth reading; and some of his successors and emulators likewise; so long as it is borne in mind that they are all, basically, wrong!