[L]uxury has two components, one physical--creature comforts--and the other social--conspicuous consumption. It is clear that the first aspect of luxury, personal comfort, should have no effect on collective solidarity. For example, the wealthy Romans spent even more money on beautifying their dwellings than on the banquets. They constructed private baths, complete with warm, hot, and cold pools, and lavished money on formal gardens. Why should baths and gardens be "enervating"? One could actually argue the opposite, by pointing to the health benefits of such "luxuries." Why should cultivating asparagus--a monstrous piece of gluttony, according to Pliny--soften the moral fiber of the Roman aristocracy?There's an interesting argument around here somewhere, although I'd say Turchin has rather muddled it up. He is, for starters, quite right to draw the distinction between wealth and ease per se on the one hand, as against inequality and invidious display on the other. He is probably right that invidious display--conspicuous consumption--reduces a sense of social solidarity.
If we define "luxury" as conspicuous consumption, however, the argument begins to make more sense. The main point of conspicuous consumption is to signal to others high status, power, or wealth. ... Conspicuous consumption is inherently divisive because it draws boundaries between the haves and the have-nots. It elicits envy and weakens solidarity. But it is even more important as a symptom of deeper processes--growing inequality and within-group competition for resources and power that gradually undermine group solidarity.--Peter Turchin, War and Peace and War 293 (2006)
But in talking about baths and gardens, he seems to have picked a poor example. For one thing, if these baths and gardens are in private homes, they are not at all the best index of "display," insofar as their very privacy is going to keep them out of the face of the have-nots. More damaging, he seems not to realize that a Roman bath was in large measure a public institution, available to a broad swath of the populace--a leveller, not an excluder.
But set aside the matter of conspicuous consumption. Social solidarity, or the decay thereof, is not the only factor in determining the success or failure of a population. For even if everyone in Rome had access to baths and gardens, stuffed dormice and pickled oysters, still there was a whole uncountable multitude on the outside looking in who weren't part of the solidarity equation at all. They just wanted the swag. The insiders, even if they are well fed and well equipped for battle, will nonetheless be reluctant to leave their life of ease.