It seems like most people are forgettong that it's pretty strongly implied that left unchecked by the Calm, Sin would kill everyone. So once they stop the possibility of a final summoning they really don't have much other choice unless they want to let everyone in Spira die. And sure death doesn't seem so final there, but most would probably become fiends, so that seems like a pretty awful fate to me. Plus there would be no new life and there can't be new life in Zanarkand anyway, as far as I know.