Modern Egyptians are a mix of ethnicities. Most Muslim Egyptians are descendants of the Arabs who conquered Egypt in medieval era. Most non-Muslims are descendants of the ancient Egyptians, though probably with a lot of Greek, Roman and even Arab genes thrown in. But it's not like the ancient people of Egypt were simply wiped out any more than the ancient Romans or Celts or Aztecs were. They simply blend in and have mingled with the descendants of those who came to their land later.