Since you've stated the indisputable facts--that Jacques Barzun is amazingly learned, a lucid and often charming writer, and that he deserves applause for the feat this book represents--let me say right away a couple of contrary things.
First of all, the premise of your comments seems to be that a book widely bought is a book widely read. Something tells me that From Dawn to Decadence is one of those best-selling books that gets browsed--at best. (Of course, it's an untestable hypothesis. Didn't our own Michael Kinsley once try to gather evidence by sticking $5.00 redeemable coupons deep inside some fast-moving books on the shelves of D.C. bookstores--never to hear from any reader who came upon them?) Barzun himself seems to have figured as much. He has outfitted his book with typographical paraphernalia that suggest he's trying to invent a new kind of literary production: a long book for people in a hurry. He's got pithy quotations from key historical figures in the margins (like the "pull-outs" of magazine articles--though he prefers to call them "add-ins," since he doesn't always incorporate the quoted words in his own narrative), and he uses symbols to point forward (>) or backward (<) to lead grazing readers to the first or fullest treatment of the topic they're passing by. These devices remind me of the pointing hands in the margins of 17th-century religious books that call attention to scriptural quotations--and I'm tempted to venture the grandiose guess that just as those were meant to guide readers early in the age of print through the unfamiliar experience of reading a book, Barzun senses that he's writing at a late stage of the print era, and wants to help out his tentative readers too.
I want to be clear: I agree that anyone who sprints through this tome without reading it sentence by sentence will miss a lot. They'll miss some excellent phrasing: "Except among those whose education has been in the minimalist style, it is understood that hasty moral judgments about people in the past are a form of injustice." Or: "The cultural predicament after a revolution is how to reinstate community; how to live with those you have execrated and fought against with all imaginable cruelty." Among my favorite moments are those when Barzun puts aside what sometimes seems to me his Cheshire-cat grin, and allows himself some expression of raw personal feeling. As you say, the most poignant instance comes in his discussion of World War I, as when he reports his childhood memory of huddling in the cellar to stay safe from the shells of the German "Big Bertha" guns firing from as much as 75 miles away. I half-expected him to revert to his cool mode of gentle irony by pointing out that we now use that name for a brand of golf club. But he didn't. So I can.
In any case, let's assume that people really do read this book from beginning to end. What will they find? What is its argument? What case does it make for how to understand a half-millennium of Western history?
Let me pick up on your shrewd comment that "a neat trick of Barzun's is to embed in his appreciation of others a defense of the standards by which he wants to be judged." Suppose we accept that invitation. The "central role" of historical writing, he says (defending Macaulay from those who charged him with being too devoted to the idea of progress) "is to present patterns and permit the welter of facts to be reconceived." But the pattern that Barzun presents is a familiar one: fragmentation followed by consolidation. The force of what he calls EMANCIPATION (using the small caps he reserves for his big themes)--that continually, if not continuously, corrosive force by which human beings break down the structures in which they find themselves constrained--is always leading to the exhilarating but also bewildering condition of newness and uncertainty. When the social and individual strains become too great, some stabilizing, often reactionary, force arises and seeks to restore a sense of belongingness by putting forward claims about ancestral glory and collective destiny. This task of giving people reassurance that they belong to some coherent plot or narrative was once performed in Europe almost exclusively by the Christian church--but from the start of Barzun's story we find the church under siege from all directions. Later, it is succeeded by various entities such as kingdoms, nations, the ideal of reason, or certain utopian visions of human freedom. None of them last--and, always aware that this process doesn't work cleanly or sequentially, Barzun is good at narrating the messy transitions from one to another.
Still, the title of his book, From Dawn to Decadence, promises a straightforward Big Story in which these small, discrete stories are contained. This is the Big Story (also a familiar one--one that I've been charged with telling and retelling myself) of general slackening, weakening, and breakdown in the Western mind. The publisher has even adorned Barzun's book with blurbs reiterating the promise that readers with a taste for Decline-and-Fall stories won't be disappointed--like that from William Safire, who calls the book a "stunning five-century study of civilization's cultural retreat."
But, in fact, the weirdest thing about this book is that it doesn't really deliver on its gloomy promise. What comes through most strongly is a sense of the astounding ingenuity and diversity of the human mind--in its literary, artistic, musical, philosophical, and scientific expressions. Barzun's theme of descent feels almost obligatory--as if he, as a distinguished member of the literary/historical guild, has to summon up his eloquence (and he does) on behalf of the particular forms of cultural expression to which he is most devoted. And so the villain of the book is elusive and abstract. In fact, he calls it ABSTRACTION--a pretty vague name for the hyperrationalistic modern dream of accounting for all nature, history, and consciousness itself in terms of material forces that can be enumerated and predicted. One of Barzun's frankest and most deeply felt sentences comes in his chapter about the Enlightenment, when he writes, "It is hard to erase from the imagination all that now fills our secular culture when it thinks of 'science' and equates it with the Only Real."
Barzun's book amounts, in other words, to a long lament for the decline and disappearance of a more complex sense of reality than science (narrowly understood--indeed misunderstood) allows. But his book might be said to be, in itself, a refutation of the charge, since it demonstrates convincingly how much vibrant protest there always has been against all reductionist accounts of human nature and experience. As for his conclusion, it may be meant to be stirring, but it comes across as straining to be cute. I'm referring to the closing pages in Part IV that you mention--where he describes the condition of 21st-century humanity as one of soul-killing boredom in a mechanistic, statistics-obsessed, bloodless world--and, not unlike Ray Bradbury in Fahrenheit 451, imagines a saving remnant of underground readers and listeners to music and lovers of art who re-emerge to save us from our own desiccating ingenuity.
Tell me how fair--or unfair--you think I've been about Barzun's semi-stated mega-plot. I'd also like to pick up on a word I used advisedly--namely, "bloodless." And let's talk about his treatment of the century just past, in which one would expect him to clinch his theme of decadence--a term he defines as meaning simply "falling off." He attaches it to our own time because he finds the present "peculiarly restless, for it sees no clear lines of advance." I'm somewhat sympathetic to this view--but isn't there also something to celebrate in our "falling off" from the religious wars and nationalist/imperialist struggles that were so devastating to everything Barzun holds dear?