Well, their marriage rule is really taking a bite out of their numbers.
I heard about a reform movement that encourages marriage to the outside.
When in power the Shah of Iran consistently met with Zoroastrian Magi [Priests] to encourage them to increase their numbers, but that all went to the trash can with the Islamic Republic.
But if they intermarried with Islamics that would do the same thing. Too bad but the Persian people are a conquered people --- conquered by their Arab masters who forced the Arab religion on them.
It's interesting that the magi or wise men in the Bible were likely Zoroastrian priests or astrologers --- followed a moving star all the way to Jesus.