Ahhh... yes, Schubert and Mozart are the obvious contenders. I'm fairly sure that our modern medical bag could have kept them going for a fair few years.
But... their output is already perfect. Schubert, in particular, had achieved his potential in the most complete way imaginable through his astonishing productivity. If we have to have league tables of composers, Schubert is already in the top two or three, every single season, for me.
What about those who showed great promise, yet to be completely revealed?