*SPOILER*
Tidus is a 'dream' of the Fate. As was his father, they're dreams that the fate have of old Zanarkand. When you destroy Sin from the inside you in essence let the 'Fate' stop dreaming to keep giving Aeons to the Summoners.
They gave their souls for a way to defeat Sin, and until Sin is gone permentally they can't die. When they 'die' Tidus being a figment of their dreams 'disappears.'
I haven't played FFX-2, but i've finished X.