Yeah, I'm with Torn here, Arceus allowing Legendaries back as pokespirits just doesn't feel like it works right. I mean if he was going to give up that kind of power the kid would have to have gone above and beyond to show how innocent their intentions were, right? And if a kid like that was to get that sort of power it would be easy to walk over them, in my mind that is.
Then we have the image of a character with genocidal preferences such as Ignis or Hailey(?) getting there hands of them however unlikely it is it is still possible. That would bring many awful events to the world.