Chicago classical music fans recall the time not so long ago when radio station WFMT was “the most cultured radio station in North America.” But “much of what made WFMT truly distinctive seems to be eroding in slow but perceptible degrees, a decline driven by the difficulties of making classical radio commercially viable but also by economic reverses suffered by the fine-arts station’s corporate parent, Window to the World Communications, which also owns and operates public broadcasting station WTTW-Channel 11. The Arbitron ratings evidence a decline of another sort…