There are many in the classical community who think that living composers are not as good as pre-20th century composers.
Well, I'm in that club. Putting it bluntly: there is NO living composer worthy of standing with the great masters of the past. There is no one the equal of Bach, Mozart, Haydn, Beethoven, Schubert, Schumann, Mendelssohn, Brahms, Wagner, Dvorak, Bruckner, Mahler, Tchaikovsky, Rimsky-Korsakov, Stravinsky, Ravel, Debussy, Prokofieff, Rachmaninoff, Shostakovich....
Not that there are many talented, hard-working, serious composers; there are. Listen to that Rihm symphony above. It's a dang powerful work; but will it last? Probably not. The Golden Age of Composition is over, let's face reality. Many reasons and explanations are given. I'll give you my top three:
1. Roughly 100 years ago composers decided music had to have social value, to mean something, to be expressive of their times. Well the 20th c had it's share of horrors, and so much of the music exemplifies this. It's not uplifting, it's not fun. It's often brutal
and not something you can listen to for enjoyment. People don't want that.
2. Composing has become too easy. Like many other arts the really hard skills have been skipped or omitted. Most painters today can't paint like Rembrandt and they don't have the technical skill to do it. They went straight to modern art where those demanding skill aren't needed or wanted. Same in music. Many composers today haven't done the hard work of learning counterpoint for example. Those old guys? The knew their basics and worked hard to acquire their technical skills.
3. The world is too noisy. Franz Schmidt used to say that a fine musical ear requires silence to develop properly. Today the world is so noisy - everywhere you go there's noise. By contrast, most of those dead white European composers lived in what must have been quiet, peaceful surroundings.
I feel really bad for modern composers; they think they have something to say, but there are very few who want to listen. I doubt that the situation will change, ever. For good music to thrive it also requires a significant audience, and that's a real problem, too.