You won't be seeing John Barrowman as Captain Jack Harkness in the "Doctor Who" 50th anniversary special for one very simple (but surprising) reason: ...
Imagine a world where you can't die. Sure, you can suffer from disease, be shot, get wounded horribly, but you can't die. You're immortal, just not invulnerable. Now imagine a world where that's the case for everyone in the world. No one can die.