As I have been given to understand it by PBS/Discovery Channel documentaries in my youth, and not making recourse to any sort of reference here: during the famous Arab conquests, Egypt was very deeply changed. The modern population of Egypt is mostly descended from the Arabs who brought their language and script and religion with them; the people who actually have the most in common genetically and linguistically with the ancient Egyptians we are familiar with are the Coptic minority, who were mostly Christian by the time of the Arab Conquest. They are the ones who have a script with its roots in the sort of cursive hieroglyphics that emerged in ancient Egypt, and Champollion learned the Coptic language and script in his attempt to decode classic hieroglyphics. When Islam arrived on the scene, the invading Arabs and the Egyptian converts had no problem desecrating their own cultural heritage, chipping away the faces of statues and the like because of Islam's edict against idolatry.

Now, I may very easily have my facts wrong, so I recommend you spend at least ten minutes researching (which I can't be bothered to do on a Tuesday morning, apparently) before you make any decisions – and the fact remains that THIS IS AN AU as I must keep reminding myself, so it's perfectly possible they a) had a kinder, gentler Arab Conquest which respected native folkways, etc., or b) they had some sort of cultural revival where they regretted what they'd done re: the desecration and vowed always to preserve what they had left.