Pokemon in Equestria. That's the gist of the result of Arceus' decision.
See, Humanity was created in hopes of providing something... more, to the Creator's world. But they grew beyond their programming, and after being punished for their mistakes, realized that if they angered their Gods, they would be struck down.
So they sealed the Creator away, in an effort to live their lives how they chose.
Fast forward a long time, and the Legendaries became not just myths, but treasures. Desirable. And with the Pokéballs, they could be tamed.
So Arceus decided enough was enough. Action needed to be taken. And the decision of mass exodus resulted in all the good in the world getting shipped off to Equestria.
He cared for his creations, for his family, and by leaving behind all of the evil... His world was left to burn.

But that's not to say travel back and forth is impossible, yet. Perhaps there's still hope for those left behind.

A companion story to A New World, a New Way.