How do the people know it's an eternal calm? They behave as though this is the first time they've ever been truly free of Sin and as though the entire world has adapted because of this. But what makes the people so sure Sin won't come back. This is only three years after Sin's defeat, when Braska defeated Sin the calm was around for 10 years, yet people stayed the same, Spira didn't change at all. I know we (and Yuna and co) know Sin isn't coming back, but why is there no doubt for the rest of Spira?