Schubert died at 31. How much music did his early death deprive us of? Not a lot.Really? And how can you tell? Gosh, I wish I had a crystal ball.
The whole article is a muddle, though he does get that we'd have lost quite a lot if Carter had died young or if Verdi had died at 78, and that Mozart had more to say. Schubert, too, I'm willing to bet.