I love Who, and I'm a big fan of Moffett and Smith, but there's one thing that just makes no sense to me.
OK, the Doctor is ancient, and he can travel to any point in time and space and has done so for at least 1000 years, probably more. His time line is not linear from our perspective, any of his incarnations can turn up at any point in time and stop you from doing that bad thing you wanted to do.
So him dying at some point shouldn't really have any effect. You can't breath a sigh of relief that he's dead, because he still might turn up and stop you. Or save you. You might turn out to be the first person he ever met, thousands of years after he "died".
He's had a millennia to go anywhere, anywhen, which weaves him so intrinsically into the fabric of space time that even his actual death won't remove him from the universe or stop him from turning up on your doorstep.