I can't help but draw parallels to decadence and the fall of Rome, was it the clash of a more spartan and stoic culture (islam) coming up against the decadence of the Sassanids that lead to the downfall of the "softer" and more "addicted to sense pleasures" empire?
How much of a role did the off-shoots of Zoroastrianism like manchienism and mazdaism etc. have to play in it's fall?
Thank you!
Just to start, I would like to point out that "Decadence" as a reason for an empires collapse is an extremely flawed Gibbonism that is no longer taken seriously in academic circles, the reasons for their fall are in reality pretty simple. The Sassanid Empire had only five years earlier ended a 25 year war with the Eastern Roman Empire that severely weakened both of them economically and militarily. In addition, a contagion of Yersinia pestis had killed at least 25% of the population of both empires. To make things worse, in the Persians case the agriculturally productive provinces in Iraq were hit very hard, losing about 30% of the population. As for the actual invasion, the Persians fought many large scale open battles against the Arabs, but were in almost all cases were soundly defeated (though the numbers for these battles are unreliable at best). What was really the end of the Sassanid empire was when its generals, the nobility and various clients and provinces began to defect, such as the Delimites, Lakhmids and Kurds. The upper classes and the administration simply lost faith in the empire to protect their interests after so many defeats. As for the decline of Zoroastrianism in Persia, it was among the non-nobility a slow process of second class citizenship and cultural dominance that slowly Islamized Persia over the centuries.
Source: Decline and Fall of the Sasanian Empire: The Sasanian-Parthian Confederacy and the Arab Conquest of Iran by Parvaneh Pourshariati
While you wait for answers to your specific question, very similar questions have been answered very thoroughly in the past here on r/askhistorians:
But to give a quick answer:
was it the clash of a more spartan and stoic culture (islam) coming up against the decadence of the Sassanids that lead to the downfall of the "softer" and more "addicted to sense pleasures" empire?
I dont think this has ever been the "narrative" in the fall of Sassanid Iran. Rather, it's important to remember that the Romans and Iranians had been fighting each other almost non-stop for 700+ years with no other 3rd parties to serve as a counter-balance on their respective eastern and western fronts. The Sassanid Empire itself was going through political upheavals and was in a weak state internally after the incredibly exhausting--financially and politically, for both sides--Byzantine-Sassanian War, the last major conflict in the aforementioned, almost never-ending Roman-Persian wars. Therefore when the newly formed Muslim army started gaining momentum--and therefore loot--it was able to attract more and more soldiers (who as converts to Islam would be guaranteed a share of the booty) it snowballed into something huge, that was able to decisively defeat not one but TWO major empires in conflict. Richard Frye covers this pretty extensively in The Golden Age of Persia.