Well, for starters we do not know that the goal of the game will be to get the heir on the throne or - at least - that it will be the ending. Chances are the story will be a good bit more twisted than that. Prophecies and general drift of events all seem to point to the collapse of the Empire, that's probably something that's going to happen in the end no matter what, considering its been around for quite a while now (yes, I do happen to be an Asimov reader) and its about time we moved on.
The actual date of the event, however, considering that its probably tied in to the main quest, can actually take place at any point whatsoever. Because you have the choice of putting the quest off indefinitely, you could well dill-dally for a few years or more.
Considering that we only have one ending availible at the end of the game, by the way, I would gather that we do have some fairly monumental and historical event occuring at the end which could not really stand being branched for the sake of historical records in future Elder Scrolls games.
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