This will be Lightning's last game and will end with a happy ending. The game has just one ending.
Well that's a fucking awful way to introduce your game. Not that I don't like happy endings but you shouldn't start off by going "Don't worry it'll have a happy ending". Kinda ruins any suspense.
Toriyama (Lead director of the XIII) has stated in past interviews that Lightning will receive a happy ending because fans thought the past two games had depressing endings. It's just Square-Enix trolling/sedating their shitty fanbase.