Decline precedes collapse a bit like old age precedes death. That would be three centuries prior. The 3rd century crisis, two centuries prior to the (2nd) sac

Decline precedes collapse a bit like old age precedes death. That would be three centuries prior. The 3rd century crisis, two centuries prior to the (2nd) sack of Rome (1rst one was 8 centuries before that), already looks like a stopped, and partially recovered from, systemic collapse to me. (Also even collapse isn't instant - Rome got sacked 3 times in a span of 62 years !) I'm not progne, so I would rather say that the cycles are about growth and decline, with "collapse" being notable because things can decline much faster than they can grow (again, consider gestation - birth - early years vs death, especially accidental death). (And yes, business cycles are also like this, on a shorter timescale.) As for guessing were WE are at, consider how the combination of growth then of decline also results in a semistable brief peak : we seem to have had it in the 90s. Unless of course you also consider the (ex-)URSS, which had a first collapse at that point, at which point the peak was probably more something between 1945-1970-1991. But then previously we were exclusively talked about the Western Roman Empire, while the Eastern Roman Empire survived for another millennium !! (Including recapturing Rome, which then was sieged/sacked two more times.) I guess you might want to look into that for counter-examples ?