Haven’t looked at that but it’s not clear how one can “debunk” a vague saying to begin with.
Cycles in history is of course not "of course not true". in any complex system, there are oscillations. Most famous is economical boom and boost cycle.
Meme is not without merit. The reason we are able to discuss things like gender/race equity in marine corps promotions is precisely because these are good times. And this lack of focus may lead to bad times eventually.
> Ibn Khaldun, 14th century Muslim scholar from Tunisia
My point is: the dog whistle/modern meme is basically a reference to Spengler and others, because their ideologies align relatively well with the patchwork of neo-fascist ideas. That's what they hint to and promote. That's why I explicitly mentioned Spengler and the 20th century. Lot of other people developed some form of a cyclical history but that's not really relevant here.
which historians? And why should we pay any attention to their consensus?
Ascribing any fixed time scale to the cycle is going to be fallacious as it depends on environmental factors.