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.