Practically, no one can stand aside from a sweep as deep and broad as the decline of our civilization. The adverb "practically" makes the claim indisputable. One could dispute that our civilization is actually in decline or that the claim is even intelligible. Or (as I do) one could dispute particular aspects of the decline's diagnosis.
I don't balk at Weaver's claim that nominalism is the real culprit and that the Renaissance is somehow complicit. I don't quite understand, though, his blaming Aristotle, Thomas, and the Catholic Church for emphasizing that virtue (in a way) is not concerned with the extreme. Ideas are extreme and virtue reaches toward ideals. The idea of moderation or compromise is extremely repugnant to Weaver.
He likes Thomas's teaching on language but not on virtue. One could conclude either that he's eclectic or that he's thinking for himself. My judgement is that Weaver thinks for himself but that his thoughts lack a certain unity. They are beautiful and attractive, yes. But do they cohere? Could they make a real world?
For me, the virtue of Weaver is that he reminds us of ideals. He alerts us to the deadening effects of materialism and "realistic" approaches. I owned this book for many years but never read it until recently, when it was mentioned in a history of conservatism in American. It is good to know that conservatism and idealism are partners.