A really interesting book that follows this line of thinking (although not re: Sparta, but instead the UK and the Dutch empire) is First Class Passengers on a Sinking Ship: Elite Politics and the Decline of Great Powers that lays out exactly the process you've outlined here. Build off of Arrighi's The Long 20th Century but includes a lot of inter-elite conflict that leads to paralysis and collapse. As an elite class consolidates power and stops adding to its numbers, relying more on hereditary than meritocratic positions, etc etc it all goes downhill. We're in exactly that stage now. Absolutely love your characterization as the American Crisis of the Third Century, totally fits and will be stealing it thanks!
A really interesting book that follows this line of thinking (although not re: Sparta, but instead the UK and the Dutch empire) is First Class Passengers on a Sinking Ship: Elite Politics and the Decline of Great Powers that lays out exactly the process you've outlined here. Build off of Arrighi's The Long 20th Century but includes a lot of inter-elite conflict that leads to paralysis and collapse. As an elite class consolidates power and stops adding to its numbers, relying more on hereditary than meritocratic positions, etc etc it all goes downhill. We're in exactly that stage now. Absolutely love your characterization as the American Crisis of the Third Century, totally fits and will be stealing it thanks!