The Arsacid state was essentially a balance of power between the 7 Great Houses, chief among which were the Qarenids, Ispahbudans, Mihranis, and Surenis. The Sassanid state was not so much a revolution against that system, but the Parsiks (Persians) joining in the sharing of power. The 7 Noble Families were the frame of the Iranian nobility, acting as Kingmakers in the Iranian plateau. The Sassanids had their economic base in Mesopotamia while their military and power base were in the Iranian plateau, derived from their delicate relationships with the Noble Families. Those noble families monopolized official appointments for centuries. It was previously believed that the Sassanids greatly reduced the power of the Seven Great Clans, and reinstated Parsik dominance in Iran, but recent scholarship shows that they slotted into the Parthian system very cleanly. The Sassanid military was in large part drawn from the Seven Clans, who received a guarantee of security in exchange for recognizing a Parsik Shahanshah. The Persian status of the Sassanid State is greatly overstated in some respects. This was, after all, a state that prayed in Avestan, read in and laughed to Greek, governed in Aramaic, and fought in Parthian.

This relationship broke down shortly before the Arab conquests. The Persian King Khosrow Parviz was overthrown by his son Kavadh, with the support of the Seven Clans, but his brutality alienated them, and turned them hostile. Due to his being dependent on them for his kingship, this swung the balance of power wildly towards the Seven Clans, and the delicate Iranian state popped like a balloon. The next Shahanshah was unsuccessful at piecing the country together, and by that point any unified Iranian state was effectively dead. No man could rule Iran by himself; any pre-Islamic Iranian Empire, and even some post-Islamic ones, found itself heavily dependant on the support of the princes of Khorasan and Adurbadagan (Azerbayjan), because they secured it's power in the Iranian plateau while the dynasties themselves ruled in Babylon/Ctesiphon/Baghdad

Thus, when the Muslims came marching in, they found themselves battling a series of small principalities rather than any unified Iran, and if Iran had been unified, then the Muslim's success would be far less certain. The Seven Great families indeed, continued to exist after the Islamic conquest, which really only ended the Parsik grip on power, for a while. The military structure of early Islamic Persia was not too different from that of Sassanid Persia. The Qarenid dynasty continued to have sporadic hostility to the Caliphate, and the Mihrani dynasty slowly Persianized and eventually formed the Samanid state. Iran, for it's own geopolitical reasons, converted en masse surprising early into Islamic rule, and it was during this time that Persian really came into it's own as a language of culture. The reasons for this are not quite clear, but it is true that the Arabs incorporated many Parsik styles into their new governance. The whole of the Iranian plateau, previously cleaved into a hundred different Iranian peoples and ethnicities, began to Persianize. This is similar to how Latin only came into it's own as a language of literature after the fall of Rome. As a result, Persian flourished, finally replaced Greek as the lingua franca, and experienced an extremely successful revival, and it was this, that finally ended the story of the 7 Clans.