Just because "classical" does not refer to the historical period, it still applies to Ligeti, Debussy, Stravinsky, Sorabji, and even Glass. Sure, the sound and stucture of the music has changed completely, but it still is "classical".I don't see why this is a problem. Why would you want to make things more troublesome than they're worth? If it bothers you to call Ligeti's work "classical", then you might as well just refer to it as "music". There, doesn't that make everything easier?