I think it's hard to tell if Mozart would have the standing he does today if he had died in his 60s. Part of the Mozart "glamor", and part of the reason he overshadows Haydn, is the fact that he died at a tragically young age... which is not to dismiss Mozart's greatness as a composer. But the thoughts of his early death and the fragility of life are things we can't put out of our minds when we listen to his music. It's a little like Keats in literature. I think if he had lived on, his music would probably have begun to sound a lot like Beethoven or Schubert. But as with other counterfactuals, who knows.