Yeah but that isn't really the point. Zoroastrian temples were being maintained in the countryside, yes, and there was one in Baghdad, but that didn't mean it was a real force. The Nobles had largely abandoned the faith, the urban population had also done the same(Rayy, for example, was in a transition from Zoroastrianism to Islam at the time of Harun al-Raschid. The Samanids were efficient proselytizers and they didn't seem to have much trouble with Zoroastrians among them.
The areas where it did survive, Yazd, Mazandaran, and Gurgan, as well as the countryside of the Iranian plateau were of course still majority Zoroastrian until about the time of the Mongols.
Look at the discussion on the previous page. There doesn't seem much likelihood of the Shah being deposed; he's a good figurehead to keep some semblance of the state alive and tradition is tradition. Since the Sassanid state depended very much on honors and that sort of thing, the Shah could in a sense be like the Japanese Emperor during the warring states era. There could be another ruler attempting to create a new dynasty, but the Parthians are unlikely. The Armenian branch had to compete with the very successful House Mihran, hardly interested in letting them take over the empire.