I don't think this is true at all. We are only talking about a few 80's songs, and K-Earth playing something in rotation that just also happens to be on JACK or KROQ is not going cause any demo change for either. JACK also plays 90s and Currents which K-Earth does not play.
That said, WCBS FM is in a very different situation, and CBS plans to play songs that charted in New York as well. I don't think there is any comparison between the two. Different markets, different stations, different audiences, different heritage entirely.