I think there is such a thing as the curse of the ninth. But it's not that it kills composers. It's that if you get around to a ninth, your symphony is inevitably going to get compared to Beethoven's ninth, and there is no way to win that competition. Especially if you decide to do a big, choral thing for your ninth. It's all very unfair.
Shostakovich escaped this problem by (perhaps quite deliberately?) writing that bouncy, jaunty little piece that we have all come to love. But now he has worsened the curse, because if you decide to follow suit and make your ninth a lighter work, you get compared to Shostakovich. And you can't make a big serious work either, because then Beethoven knocks you flat.
And thus, modern composers are well served to not write a ninth at all. Or to leave their symphonies unnumbered. Or, if you're going to write a ninth, you'd better also write a 20th, or 67th, because that way you show that you conceive of the symphony in a completely different way and thus you can escape the unfavourable comparisons. Except of course for getting compared to other prolific symphonists.
In short, the longer the classical tradition goes on, the bigger the shoes that new composers have to fill. No wonder more and more talent gets siphoned off into pop.
