If you believe Narshakhi's "History of Bukhara" (as referenced by Mary Boyce's Zoroastrians), Muslims often had to bribe Zoroastrians to come to Friday prayers. Zoroastrians were slowly dwindling throughout the Caliphates, but the deathblow came after Hulegu Khan invaded Khwarazm. If you make the Umayyad or early Abbasid Caliphate implode, or at least much weaker, a Zoroastrian Persia could rise again.
The Abbasid Caliphate gave plenty of incentives for Zoroastrians to convert. Muslim converts in Zoroastrian families received much greater inheritance rights. Fire temples and other sacred places were destroyed or converted into mosques, gradually uprooting the religion. Dhimmis had to show public submission before the ruler. Even with all of this happening, new Zoroastrian works like the Denkard, apocalyptic literature, and the Bundahisn were being created. The sacred fire Adur Gushnasp lasted at least until the middle of the 900s. Zoroastrianism was down, but not out.
A different Shia movement could fulfill the challenge. According to Shia legends, Husain had at least one child by a Sassanid princess. If the Shia'tul Ali gathers more followers in Persia early on, it could have more syncretism with Zoroastrianism.
Ehsan Yarshater's works such as the Cambridge History of Iran are referenced here (via footnotes from an old research project on Zoroastrianism).