Wells uses an oxymoron, the juxtaposition of the words "ruinous" and "splendor," to underscore the reality that the Eloi society is not truly a "utopia," but rather a degraded vestige of its former glory.
When he first arrives, the Traveller describes the Eloi civilization:
As I walked I was watchful for every impression that could possibly help to explain the condition of ruinous splendour in which I found the world—for ruinous it was. A little way up the hill, for instance, was a great heap of granite, bound together by masses of aluminum, a vast labyrinth of precipitous walls and crumbled heaps, amidst which were thick heaps of very beautiful pagoda-like plants—nettles possibly—but wonderfully tinted with brown about the leaves, and incapable of stinging.
By contrasting these two words, Wells creates a sense of irony and suggests that the society of the Eloi is beautiful but ultimately doomed to self-destruction and "ruin." Though once "splendid," the Elois' society reveals humanity's failures, rather than its success. While the remnants of this civilization display the "splendors" of humanity's triumph over the "stings" of nature, the demands of physical labor, and human susceptibility to disease, their decayed structures represent the degradation of humanity's collective potential. The oxymoron "ruinous splendor" serves to convey a greater theme in the novel—the idea that a society that ceases to value curiosity and progress will devolve and stagnate. Amongst the Eloi, the desire for comfort and safety has displaced progress, to the detriment of their civilization.
This oxymoron also serves to illustrate the contrast between the Eloi and the Morlocks. The Morlocks are evidently "ruinous," an "inhuman" race that feeds on its "brothers." But the Eloi, though they retain their beauty and civility, are also ruinous. By abandoning the project of social and political progress, the Eloi have lost the strength and intellect that once defined humanity.