And yet the Arabs did just that OTL. Supposing Nestorian or Manichean Christianity became prevalent enough in Arabia -- would that make Christian Persia plausible?
Did what otl? The Lahkmids (the only Arab polity in Iraq and with close proximity to Persia) did not conquer Persia, and for the most part were pro-Sassanid. The Assyrians had no part in the conquest of Persia, and had not had any political ambition since threaten Achaemenids, so I don't know what you are referring to.
If Arabia is united under a single country like otl caliphate, and they invade at an opportune time then it is possible for them to conquer Persia. Once they do they would have to enforce strict rulings against Zoroastrians and assimilate Persian culture to their administration, possibly removing Pahlavi script and writing Persian in the Syriac script, kind of like otl Abbasids. However if the Christian rulers behave like the Umayyads, it is not for sure that the Persians won't rise up and remove their Arab rulers.
With that said, I feel like it is unlikely that this happens as Arabia remains politically fractured and is plagued by tribal warfare. What is more likely is a constant decline and rebirth of the Persian empires at least in the foreseable future, all building on the nostalgia of the Achaemenids.
