A better POD would be the Plague of Sheroe not killing Sheroe (Kavad II), which predictably threw the entire empire into chaos because the man had already killed all of his brothers. Even if the empire was bloodied in the 602-628 War, the Sassanids would (probably) not have collapsed if they did not sink into the 628-632 Civil War.
We could look to the Byzantine internal wars (Doukas-Romanos, de Bailleul's revolt, etc) after the 1071 Battle of Manzikert as a comparison. Manzikert itself was a lot less horrific than what came after.
Besides, a Roman-Iran alliance was already in place during the Rashidun Invasions. It just didn't accomplish its intended purpose.