When Sin is destroyed, the Fayth remains; however, when Yu Yevon is destroyed, the Fayth cease to exist and instead are sent onward to the Farplane. This causes summoners to no longer be able to summon Aeons and brings about the Eternal Calm. It also causes both Auron and Tidus to disappear, as Auron is an unsent guardian and Tidus is a dream of the Fayth. Once the Fayth stopped dreaming, Tidus stopped existing.