We know that classical predominantly appeals to seniors now.
Well that's who still listens to AM radio so classical might be one of the few formats that can still get an audience. There are lots of people in that age group whose hearing has declined to the point where AM sound quality is probably fine for them, and some listen through hearing aids anyway. Other than that, at least the AM signal gives the station an on-air presence and brand awareness in the market and the hi-fi seekers may be reminded to fire up the stream if not HD Radio.