The Zoroastrans were in a death struggle with a young Byzantium during Heraclius's rule. Had they had their way, Christianity would have been extinguished 1500 years ago.
Heraclius took the shattered remains of the Byzantine army, drilled the snot out of it, and, ignoring all other strategic targets, drove straight for the sacred city of the Zoroastrans. He sacked it, extinguished their sacred eternal fire, and tossed dead livestock into their sacred lake.
The politico-religious crisis that this caused brought down the Sassanid Empire for good. The Zoroastrans have been on the ropes ever since - good riddance if they finally die off.
But then, don't you think that had the Zoroastrians sustained themselves, there would have been no Islam, no Iran(the Religio-Islami kind it is today). So the real choice is: Islam or Zoroastrianism?
Look, I am commenting in a very premature manner, with no proper hold on the facts of history, except what I scraped out from various sources. I am just saying what I think, and that is not based on solid knowledge. So feel free to educate me.