Many idol group in the past already disband or have many member retiring. Its not about age but era.There's only one major plot inconsistency here that I see, and it's: How the hell did this alien think that a child would have a better chance at getting risque idol pictures than a full-grown adult?!
I also agree that the reason doesn't really matter on this one. There is no revenge goal or anything of the sort.I think it's fine that he got sent back for a stupid reason because the reason doesn't matter. The whole story has been about what he's doing with the new chance he got instead of figuring out a mystery.