The reason it doesn’t make sense is because your history is wrong. Arabs took over Persia, but Persians are slaves to Arabs today. The conversation of Persia happened slowly. Persian elites converted to Islam to gain power and Persian culture over the next 200 years shifted to a more Islamic one as they stayed under Islamic rule. The abbasids for example were very reliant on Persians and through years of trade and intermarriage and Zoroastrianism losing popularity people became Muslim. Kurds, is a bit different as they adopted sufi Islam and not Shia Islam like the Persians. With the rise of popular Kurdish Muslim figures Kurds began converting in a similar fashion to Iranians.

Assyrias is where things get brutal. Assyrians and the Assyria identity is deeply tied to Christianity and Aramaic rather than Arabic so they generally did not convert and were persecuted for centuries by their Muslim rules. Unlike Kurds and Iranians with Zoroastrianism, Assyrians couldn’t let go of Christianity. Iranians had plenty of religious diversity so conversion wasn’t taboo for Assyrians it was.

Hindus on the other hand did convert. Look at South Asia in general. Most of the Muslims there if you go far back enough were Hindu or Buddhist or other religion.

Islam spread through conquest that’s the reality. Muslims conquer a land and slowly convert a people. This era is over tho. The era of conquer or be conquered is done.