Robert P. Morgan's Twentieth Century Music and The Rest Is Noise by Alex Ross are two standard books on the modern symphonic music acceptable repertoire.. What is new (meaning 1930s 12-tone atonality) and what is acceptable to a large symphony are listed, composer by composer, in a knowing, almost arrogant tone. The rest is noise, they tell us. Soundtracks, which introduce tens of millions of movie attendees to instrumental music, aren't good enough, aren't real art, have been poisoned by collaboration with a director and, worse, a producer. We can't have that.
So orchestras have stuck rather closely to the works mentioned in Morgan's and Ross's books. How is that working out with modern municipal symphonies??
Or would you prefer Honolulu? Its symphony already went bankrupt. Looking nationally, the writer found that symphony audiences are growing for and only for those over age 75!* The author of the link below used to serve on the board of the Honolulu symphony, so his attitude should be regarded as probably typical of the organization. What is atypical is the thoroughness with which he looked at the problem. But he didn't touch the root of the problem – low quiddity –low levels of melodic excellence.
Notice that these reviews tiptoe around the problem of the actual programmes and the annoying sound of the actual works being performed. Some symphonies have retreated, reluctantly, to the old “war horses” like Tchaikovsky, Liszt, Rimsky-Korsakov, Dvorak and others of the romantic period, but it is a forced march started rather late.
* so the growing segment of the market were adults already, before rock and roll came along. They are profoundly unlikely to be enthusiastic about adding soundtrack themes to the repertoire. These mamas don't dance, and these daddies don't rock and roll!