I think we're stuck with Shostakovich as the last composer to write a great symphony. Michael Tippett is also a candidate, but he doesn't have Shostakovich's stature.
I hate to play the blame game but I think it was the cultural revolution of the 1960s that sealed the symphony's fate as an art form. The dominant culture in the world has been American and the symphony has not been a part of that culture — at all.
It is difficult for those of us born after the 60s to appreciate the 60s. The series Mad Men helps. It addresses the fury of the 60s in an oblique manner but it addresses the rise of television and the culture of consumption directly.
The cultural content that came out of the revolution has been controlled in a manner that is different from any that came before it.
Every symphony was written in a particular culture and that culture was usually on a short, autocratic, leash. The power in control was obvious and total. There are some exceptions. The late Habsburg Empire appeared to be somewhat laissez faire along with the 19th Century German fiefs.
The culture that arose in the 60's and that has continued is also controlled, but it's controlled by profitability instead of a ruler with a specific ideal to advance. An autocratic ruler has an agenda and will use culture to push it.
The dictators of the 20th Century are the clear example here. These fascists were complete monsters, but they created some astounding culture.
The culture we have now is pushed by creating revenue. I'll admit I enjoy The Voice but everything on that show is designed to create revenue. Everything.
The symphony has rarely been an economically viable undertaking. Few composers wrote symphonies with the idea of cashing in.
Emperor Joseph paid Mozart's bills because Mozart's music enhanced his status as a ruler. Mozart was on the hook to please Emperor Joseph. Joseph had a vision of the world he wanted and commissioned music to support it.
There is no longer an emperor with a vision. We are all the emperor now. We are all the "philosopher king" and we are able to choose our individual life's culture from a variety of economically viable options.
Which vision do we choose? Apparently one without new symphonies of great stature.
There is a stunning lack of academic rigor in this post. I think there is some truth in there but I don't have any sources to cite or studies to reference except my own experiences and ideals — and I can say that because, like you, I'm a philosopher king.