Since we're having this conversation on the Philadelphia board, let's consider Philadelphia. It has a very successful alternative station, it has a successful active rock station, and a successful classic rock station. So would there be room for "classic alternative" in Philadelphia? Perhaps using your definition, Philadelphia is also "overradioed."
You need to look at more of the landscape than just Rock radio. A Classic Alternative station would be generally male-targeting. You have WMMR and WIP pulling in some massive male numbers, with WMGK and WPEN also grabbing very large chunks. WRFF, despite fairly solid/steady ratings, has tough times making money because agency buys go to WMMR/WIP etc. first to reach the largest possible audience, leaving WRFF with the scraps. Throwing in another male targeted station would only fracture the revenue further. So while I would love a good Classic Alternative station in Philly that payed homage to the music played on WDRE and Y100 back in the day, I have a hard time feeling it would be financially viable.
Could adding new wave to the mix provide some format definition to WBEN?
BEN has weaved through the lanes for years, depending on how the trends go. They've leaned on some Classic Alt before, and have weeded that back out of the playlist to lean more 80s to go after WOGL. Throwing some more New Wave into the 80s mix right now could work, but that's up to them to test/research