If one is playing a game in which situation X can happen and anytime X does indeed happen the results that produced X are changed or discarded then it has been decided that result X will not be a possibility.
If we say that result X = character death and result Y= the party survives and we eliminate possibility X then we are left with a predetermined situation Y, the party survives.
We don't know all the events that will take place but we do know that the party will make it through alive. That is predefinition of a kind.
I think I agree to some extent. At least on the micro per-encounter scale of things. It doesn't offer any predefined ending on the macro scale of will the PC become a king, pauper, god, or ooze.
If I say that in my game, PCs don't die, that rules out an outcome of combat.
I would never tell players that, however, as that eliminates a tool from my arsenal.
What muddies the water in no death = "predefined ending" further is that I don't know that I would "save" a PC until I was in an encounter where he needed saving. At that point, I would consider questionss like:
did he have it coming to him?
is there a rational alternative that doesn't look totally cheezy?
will this screw up the storyline in unrecoverable ways?
If I kill Aragorn at Helmsdeep, is that going to screw up my story line? I know Aragorn will be mad, as his storyline will end. But I've got this Sauron thing, and a ring-trek to try to run. It's early on in the campaign, Boromir's player got over it, though running side quests for Faramir until he comes on the scene is getting tedious. If I kill him, it'll cement in the players mind that there is no plot immunity, especially useful with the jerk who's holding the ring...