You have a point, but I don't think King would have had the glass stored in the two towers if he hadn't intended for it to be destroyed, or at least buried for a long while...
The rainbow might find new owners, but I think they're very patient as to when this happens. They can sit idle for years, even centuries before they roll into some poor blighter's path (so to speak). And I'm not sure prophesy was amongst Black 13's powers. (Some of the orbs do have that power though, but Black 13 is all about the todash I think.)