One of the things that need to be considered is that we are not talking today's Zoroastrianism which has dealt with somewhat of a same issue as the Jews - cultural, and sometimes physical isolation throughout the centuries. If there is no isolation (resulting in smaller communities that create their own set of values that is not always correspondent to those of society at large), and there is a perception that Zoroastrian state is a major power, religion becomes less of a mean to identify oneself as separate from the rest of the world, and more of a cultural background.
Second, there is the fact that Zoroastrianism as the state religion of Persia pre-Arab conquest was in need of serious reform, organization-wise. The "Magi" (priesthood) became basically a hereditary order that guarded their privileges zealously - since there seemed to be little ability for one not born of the Magi to become a part of the order (unlike Christianity or Islam, where path to becoming a cleric is relatively easy - especially in the era we are talking about), it lessens an impact of Zoroastrianism being anything but an "official" religion - less of an-all permeating religious and spiritual philosophy that is available to all, but more of a very elitist system that resulted in elites being Zoroastrian, but anyone that was not either an aristocrat or a magus having a very faint idea on what was going on - even less so than a commoner would with Christianity or Islam, where there was less of a mystical aspect and the holy book of religion (Bible and Qu'ran respectively) was able to be read by anyone literate - with Zoroastrianism, while my knowledge of it is incomplete, it seems that much of the sacred text etc was basically guarded by the Magi who did not want the word to get out.
So, IMO Zoroastrianism of XXIst century if it remained official religion of Persia would have probably been a very different faith from the one we know of. I am thinking there are few factors that an Arab defeat would make a difference with would be:
1) If Persia remains a major power, it will have its own imperialistic designs on the entire Achaemenid empire of old. As such, more interaction with the West, less of a "cultural isolation" when Dark Ages come to an end - higher technology level, larger territory, and earlier sense of nationalism are all likely (although AFAIK the Persians/Iranians always had a much greater sense of nationalism due to their recorded history and achievements they are proud of going for over a thousand years before the Arab conquest).
2) If Arab conquest is successful elsewhere as in OTL with the difference that both Zoroastrian Persia and Christian Byzantium, expect a large mess in the Middle East - a three-way war going on pretty much with no end in sight. A side effect is that every steppe horde that comes rolling into the Middle East (I am thinking Turks and Mongols here) will first get to Arab Baghdad (if it is stil Arab), then to Persia - which is good news for Byzantium, which is likely to survive for much longer thanks to that extra unexpected buffer.
3) How would an Arab world without Persia develop - would it still experience an early conquest-driven "golden age" of sorts and then fall into seclusion, isolation, and decline as in OTL, would it be potentially driven back into Arabia with not one, but two major powers opposing it from the point one, or would it actually benefit from having a smaller, more compact sphere of influence that is easier for one or two states to hold together - longer-lasting Caliphate that actually manages to be a real nation, not a largely meaningless title as it seemed in the final days of the Ottomans.