The referendum is about national identity, a realisation that we are our own people, no longer needing constitutional dependence on a foreign head of state. It is about casting off the last vestiges of the "cultural cringe" that clung on until only a short while ago. It is about the constitution maturing into a document that better reflects what Australia is today.
You claim we are offered a president elected by a two-thirds majority of parliament because we can't trust ourselves not to vote Kylie Minogue into office. To elect a president by the majority of the people would give that president a greater mandate than the parliament itself to govern. The point of the president is to keep the parliament in check, and vice versa.
Australia does not need the Queen any more than she needs us.