“The correct answer to your question is that nothing happened to the people who did not accept Islam, except that they were subject to a special poll tax called the jizya and that was it”—Nothing happed except a pole tax, no, they were subject to Islamic rulers, Islamic laws, and Islamic beliefs. Most of the so-called Arab Countries today, the indigenous people had their own indigenous languages, their own beliefs, their own leaders, their own laws. Do you think they gave all this up voluntarily.
The ancient Egyptian language is attested in Egypt for over four thousand years, from the appearance of Hieroglyphic writing around 3200 BCE, until it was replaced by Arabic after the Arab conquest of Egypt in 641 CE. The indigenous people, Coptic Christians are still persecuted and the discrimination against Coptic Orthodox Christians are historic and widespread issues in Egypt. They have been under siege for 1500 years.
The oldest known Berber inscriptions date back to the 4th century BC, but Berber-speaking people have lived in North Africa since at least 3,000 BC. Only recently have the indigenous Berbers of Morocco been allowed to teach their own language in schools. After the Arabs completed the conquest of Egypt in 642, they started to raid the Berber (Amazigh) territory to its west, which they called Bilad al-Maghrib (“Lands of the West”) or simply the Maghrib.