Earlier quoted context omitted.
Uh — Roman Empire lasted in part something like 1500 years and imperial China around 2100 years. Average implies that it’s not a hard and fast rule they die at that particular point — as the following sentence implies.
That's true, but those empires also had periods of traumatic decline and upheaval. Given if you grant those entities continuity for those time scales, there were absolutely periods in the middle where everything went to shit. Using a 1500 year number for Rome includes having half the empire drop off and die. The more important idea isn't the 250 years (I agree), the idea is that it's not unreasonable to model aspects…
My point was that decline and regrowth, or even political transition as with Rome, happen frequently — not just decline followed by collapse (or “death”).