Here's the challenge and the plausibility check, have the Persian empire of Cyrus turn into a southwest Asia "eternal empire" like China.
Can dynasties be toppled and replaced by invaders? Sure.
Can the realm be divided into warring states for periods lasting over a century at times? Yes
But must giant Persia be an eternal "thing" that reestablishes unified control over at least Iran, Afghanistan, Pakistan, Mesopotomia, the Levant and a majority of Anatolia, except for interruptions of at most 2 centuries? Yes-
Different but parallel idea:
Have India be under a unified state more often than not (like China) for at least the last two millenia.
Can dynasties be toppled and replaced by invaders? Sure.
Can the realm be divided into warring states for periods lasting over a century at times? Yes
But must giant Persia be an eternal "thing" that reestablishes unified control over at least Iran, Afghanistan, Pakistan, Mesopotomia, the Levant and a majority of Anatolia, except for interruptions of at most 2 centuries? Yes-
Different but parallel idea:
Have India be under a unified state more often than not (like China) for at least the last two millenia.