So it's Buddhists stealing the idea of the Kali Yuga from Hindu scriptures.
I have often considered not without wonder whence arises a fault, which, as it is universally found among old people, may be believed to be proper and natural to them. And this is, that they nearly all praise bygone times and censure the present, inveighing against our acts and ways and everything which they in their youth did not do; affirming too that every good custom and good manner of living, every virtue, in short every thing, is always going from bad to worse.
(...) and they remind us that there were no murders in those days (or very few at most), no brawls, no ambushes, no deceits, but a certain frank and kindly good will among all men, a loyal confidence; and that in the courts of that time such good behaviour and decorum prevailed, that courtiers were all like monks, and woe to him who should have spoken insultingly to another, or so much as made a less than decorous gesture to a woman. And on the other hand they say everything is the reverse in these days, and that not only have courtiers lost their fraternal love and gentle mode of life, but that nothing prevails in courts but envy, malice, immorality and very dissolute living, with every sort of vice,—the women lascivious without shame, the men effeminate. They condemn our dress also as indecorous and too womanish.
Is this from Castiglione? It reminds me of the hilarious scene early in
Don Quixote where the Don gives a pages-long discourse about the Classical "Golden Age" to a bunch of goatherds who sit around listening raptly not understanding a word he's saying.
At least in the context of Japanese Buddhism the concept has from time to time been employed to leftist sociopolitical ends, fwiw (because it's been seen as in some sense liberatory from the more burdensome and socially reactionary demands of traditional Buddhist moral theology), whereas when I hear "Kali Yuga" I think far-right "Perennialist" figures such as Evola.
MustCrushCapitalism is broadly correct about the relationship between Hindu and Buddhist cosmologies and eschatologies.