Based on my own experiences with programming that decade as a Classic Hits format, I think that's going to be the center of the universe in that format for quite some time, as it appears to attract 18-34s in almost as high a quantity as it does 35-54s.
Given that the concern among CH stations is finding the "right" 90s titles to add going forward, I think it's going to be a market-by-market decision. Some will go more Rhythmic, others more early Alternative, some more Active Rock, and others more Mainstream CHR in their choices, based on the dominant listener preferences in current music. That's practically the only logical way to do it.
As I said previously, I believe KOLA has a higher percentage of 90s songs as a counter-programming move to KRTH's strong signal in the IE, and that type of situation isn't going to be the case in most markets. So -- especially with 1982-84 being the median point in Classic Hits just about everywhere (including KOLA!) -- most stations are going to stay in that comfort zone for as long as they can.