WXRT has always had a different audience than The Drive, I don't think they'll see more than a slight slip in the ratings. It helps that 95.5 is playing a ton of artists WXRT, for understandable reasons, will never touch.
The Drive I expect to get hit harder. They've steadfastly refused to play anything after 1990 despite the window being wide open to do so and cut off potential competition. They did not do so and iHeart decided to fill the 20 year musical void.