I don't think "ending" it would bring them into a difficult situation to continue their story. They could have a new leader for the Orcs that are trying to reorganize themselves after Sauron's fall.
However, I do see your point about the "final battle". I think that's a bit too big for the player to be a decisive part in that. Mostly this game will probably be centered around Agandar/Agandaur, the Dark Numenorean, so we'll probably won't be dealing with Sauron himself at all :P