There is nothing wrong with speculation if it doesn't lead to lamenting the past - as what has happened was what was meant to happen by the qadr of Allah . But, we have to look back at history to learn from it and to better ourselves. Everything is from Allah but we have to make the correct effort. It does require noting that there were no polytheists in the Arabian peninsula, as well as northern Africa. Even in Iran, Zoroastrianism became a tiny speck of its former glory, with many Zoroastrians actually fleeing to Hindu India when the Muslims arrived during the time of the sahaba . The Qur'an gave an ultimatum to the non-Muslim mushrikeen of Arabia - become a Muslim or leave your land. We see the result with the lack of these polytheists after the Muslims conquered Arabia, northern Africa, and Persia. On the other hand, when the time for conquest of India came, no such ultimatum was given to the Hindus. Over time, Muslims and Hindus blended with each other, with only the righteous ulama staunchly adhering to the sunnah and the evil people adopting personalities that nowadays both Hindus and Muslims seek shafa'a from by visiting their shrines (e.g. Sai Baba). Even Sikhism came about because of this intermingling. So, we can easily take a lesson from this.