But you're right that the Persians did not convert folks to Zoroastrianism.
A powerful Perso-Roman Empire would have swatted Islam.
That’s right, “The Persians (particularly up to the end of the Parthian Empire) didn’t care about the religion of the conquered. The wars w/ other lands, for the Persians, was not about ‘religious domination’, nor ethnicity. They even supported freedom of religion.
Sassanid Kings such as Shahpur I was a particular friend to the Jews. The mother of Shahpur II was Jewish. Similarly, many Christians were already freely living in Iran. But, they were Nestorian & Jacobites (Eastern Christians), who were mostly of Persian ethnicity, ex-pagan or Zoroastrian & largely loyal to the Persian Empire.
In fact, for the first three hundred years of Christianity, it was in the Roman Empire that the Christians were persecuted.
The problem began later during Sassanid Empire, when war w/ Rome intensified & Christianity became the favored religion of the Roman Empire. The Sassanid feared the alliance between “Western Christians” & Roman Empire would pose a threat to them. The Western Christians were also trying hard to convert Persian Zoroastrians. That’s when the Sassanid kings established a close tie w/ Zoroastrian clergy in Iran & made Zoroastrianism (along w/ a number of related practices & social structure) the official state religion to counter the Christian-Roman alliance. Prior to that, religion & state were separated in Iran. Even so, the “Western Christian” persecution by the Sassanid was largely limited to pro-Roman Christian clergy or groups, who were perceived or actually were loyal to the Roman Empire. It wasn’t about religion per se.
I agree that a Peso-Roman alliance would have ‘swatted’ Moslem-Arab invasion of Iran & generally prevented the advancement of Islam thereafter.
It is ironic that throughout pre & post Islam history of Iran, Persian empires or dynasties, have mostly managed to dominate or break even w/ those who were their equal, but have lost to those who, one way or another, were inferior to them.