In particular, some of the more famous "Rome is decadent and about to be destroyed" complaints came just as Rome entered several centuries of expansion and power.
I don't know what the history field is like these days, but his "we don't believe those evil things anymore" asides get annoying after a while, especially when he touches on live issues. Maybe he has to lard descriptions of the past with disclaimers to ward off the evil eye, or maybe he really believes that we're superior now.
Anyhow, he holds that the trope isn't really about the "poorer, harder" people, it's a critique of decadence in the rich society. The poor "Fremen" cultures live in hard lands because they've no choice--they generally can't fight their way into richer lands, or they would have already.
OK, fair enough. Why has this critique been so popular--for thousands of years?
There seem to be two different paths. Consider a family instead of a nation--I'll get into why in a bit. A family can become so rich that they distort the economy and politics of a region, and their wealth becomes self-sustaining, requiring only modest intelligence to manage. Outsiders find it lucrative to participate, and thanks to the political distortions, competitors are suppressed. These can endure for a long time.
Another path is that described by the Chinese proverb that “You can only keep wealth in the family for three generations.” The first makes the business, the second runs it, and the third ruins it. According to the Conway Center for Family Business, 12 percent of all family businesses make it past the second generation, and only 3 percent make it to the fourth generation. That sounds like pretty substantial attrition, though perhaps changing environments plays at least as big a role as deteriorating management.
At any rate, on the micro scale we see something happening that at least superficially seems to deserve the critique--"shape up and get hungry again or the outsiders will eat your lunch." You can probably think of several businesses that were at the top of the world just a couple of decades ago, and just weren't flexible enough to make it.
Of course just being hungry isn't enough to fit you for beating the fat and lazy--it's a necessary but not sufficient condition. But it's easy to see why the trope/critique is popular. It fits, at least if you ignore the hungry failures.
I haven't done a systematic survey, but the "decadence leads to decay" trope seems to fit the smaller upstarts better than the big empires--at least in the histories I've read. Perhaps they're too small to attract outsiders to support them.
I surmise that one reason the "decadent empires" last longer than the three generations is because the empire isn't a single family, but a collection of competitors working within the framework of empire. When one crime family gets sloppy, another starts encroaching on their turf, and the overall structure doesn't change. As long as they don't start dismantling the machinery, if it's big enough it keeps going until bureaucratic friction burns out the bearings.