If Persia held, then that means no Islam reaching Pakistan, Afghanistan, or Central Asia (save through traders). Zoroastrianism would likely rank as a major world religion today. Islam might have developed a more evangelical bent.
My question is, what happens to Central Asia? Maybe Tengriism dominates? Or even an exported Zoroastrianism?
Tengriism wasn't organized like Zoroastrianism, Buddhism, Christianity, Islam etc and at least in my opinion was doomed to face the fate of all (well, the vast majority of) pagan religions and be replaced.
At that time, the Gokturks of the Turkic Khaganate were still Tengriists and strongly so, but Zoroastrianism, Manichaeism and Nestorian Christianity were already taking root, as was Buddhism to a lesser extent.
I think you'd most likely see Manichaeism or Zoroastrianism taking root amongst the Turks without Islam, more likely the latter. Buddhism would be a significant minority faith, too. Of course, the Persians would still try their luck at maintaining an Iranian population in Transoxiana and further south.