It's been discussed here many times, and such a move would drive up the median age of the cluster. Once again, the goal is money, not ratings.
There are plenty of Classic Hits stations out there with healthy money demo numbers and impressive billings. WCBS-FM, KRTH, WOMC, WOGL, KCMO-FM, WMJI, WGRR. KXSN, WRBQ, etc.
Granted, there are also some out there with disappointing revenue figures. I haven't seen billing data for WLS-FM in Chicago in a while, but I recall them being a perennial poor biller. In the past 18 months, that station has witnessed substantial ratings gains, so I'm not sure if the above statement still applies.