Of the composers who wrote after Mahler, I would say that there are some who wrote very good music, and even a few masterpieces that will make the list of music worth hearing forever.
At the top of my list would be Gliere's magnificent Symphony No. 3 "Ilya Mourometz", which is on a scale, with the power, substance and beauty of Mahler (but without all the marching). Farberman's 93 minute recording is as intense an experience as any work by Mahler or Bruckner in my opinion. (Yes, I know it is heresy to say such things on this board, but that's my opinion and it is what it is.)
In addition, I would include the following composers/works written after Mahler's death that are equal in quality (although not necessarily in scope).
Rachmaninoff - Symphonic Dances, The Bells, Rhapsody on a Theme of Paganini
Myaskovsky - late symphonies
Nielsen - Symphony No. 4
Barber - Adagio for Strings
Gershwin - Rhapsody in Blue
Hovhaness - Symphony No. 2 "Magic Mountain"
Tubin - Symphonies 3 & 5
Ralph Vaughan Williams - Symphony No. 8
David Diamond - Symphony No. 3
There are surely others that haven't popped into my head this morning.
I have not heard much by living composers, but of the dozen or so that I have heard pieces by in concert, I have especially liked Stephen Paulus. His symphonic work called Sea Pictures (not yet recorded) is very fine. I've also liked some of the music I've heard from Joan Tower.
Like you, I have little interest in cacaphony and noise without point. But I don't want composers to write more music that sounds like Mahler and Bruckner. We've already got more than 20 works by those composers to listen to. They were who they were, they wrote what they wrote. I want to hear voices that are just as compelling and just as immersive, but unique.
Saying that no one since Mahler and Bruckner are great composers is like saying no one since Shakespeare is a great writer. I don't want more writers to write like Shakespeare. I want more writers who write great literature in their own voice. And many have.