
Based on the dataset he and his colleagues compiled, civilizations big enough for internal dynamics tend to exhibit these characteristics during their stagnation phase:
* stagnating or declining real wages
* increasing rents
* decrease in social cohesion
* increasing inequality
* increasing urbanization (mostly due to no rural opportunities)
* more people pursuing education, arts and crafts in hopes of joining the elite. This sounds as a plus, until you realize it's caused by inability to keep a decent standard of living in traditional occupations
* sharpening intra-elite competition, leading to a gradual abandonment of previous norms. Dirty tricks become more and more commonplace
* crisis amplification - non-issues turn into crises, challenges turn into disasters
* 'overproduction of elites' leads to increasing corruption and rent seeking
* stagnating wages and increasing corruption leads to strain on public finances
Turchin's contribution lies mainly in providing historical statistics to back these claims, the novel mechanics of 'elite overproduction' and 'elite aspirants', and overlaying a secondary generational cycle, when a clique of elites keep power for too long (30-50 year cycle, think of Boomers in US or WW2 veterans in USSR).
(1) https://www.goodreads.com/book/show/8778747-secular-cycles
In particular, Turchin predicted in 2016, that 2020 would form the peak of social conflict in America. This strikes me as eerily close to what has actually happened…
[0] https://slatestarcodex.com/2019/08/12/book-review-secular-cy...
[1] https://slatestarcodex.com/2019/09/02/book-review-ages-of-di...
Because these companies have the highest status and social justice outrage is a form of intra-elite status competition. You'll find that this stuff becomes more intense in academia as you move up the university prestige ladder.
Peter Turchin has written a lot about the general dynamics of intra-elite competition and elite overproduction: see https://www.amazon.com/Secular-Cycles-Peter-Turchin/dp/06911... for example
All of which to say, you could liken periods of high wealth concentration as lending to decadence, though I'm not sure how useful that would be as the same argument could be made of golden ages. Decadence itself doesn't seem suspect but I would expect it to be around before things go awry.
edit: here - https://slatestarcodex.com/2019/08/12/book-review-secular-cy...
