What happened to humans that died in Childhood's End, either before the arrival of the Overlords or after? Did death allow you to join the Overmind?
It's been some years since I read the book, but as I remember it, only the chidren were part of the overmind. Old style humans weren't. And if dying allowed old style humans to join the overmind, there wouldn't have been the reaction described in the book - which seems based on a permanent separation. If it was just temporary, it would be unlikely to to create the reaction described.