The game basically ends after you beat the Elite Four. The Post-Game Chapters are just extra fluff.
Viewing post in Questions about the end (spoiler)
Hmmm...that s bit odd to consider that but nevermind.
The game was great and i got a lot of fun
.PRos : Good map/huge pokedex/Humor
Cons : Difficulty drop too much at the end
Will u add some add ons in the future or do you consider the game finished ?
It would be relevant to add ( IMO) a little arena with high skilled trainers since legendaries have been released on the island.