Why all the "Gloom and Doom"? End of the World prophecies? Why?
Morrigan and Flemeth could equally as well be trying to save the world as destroy it, you know.
What, I as you, is stopping Bioware from Pulling a similar stunt as ME 3 did? At the End of ME 3, Shepard is supposed to die to save the galaxy. Bioware pulled a stunt, and if you A) chose the correct ending, and B ), gathered enough War Assets, Shepard would live. Again I ask: What is stopping Bioware from pulling this in Dragon Age?
The OGB will not appear (beyond a possible Codex entry or 3) in Inquisition. But nothing has been said about the next game (or a fifth, if certain rumors I've heard about DA being 5 games long is true), assuming of course that the DR was done.
Possible Scenario: Final game boss (Archdemon level from Origins, Merideth/Orosino from DA 2, etc) requires the protagonist to die, BUT, if the Warden did/allowed the Ritual, the Man that grew up from the OGB would then take the sacrifice instead, which would have been Flemeth and Morrigan's plan the entire time (and I consider both to be manipulative B****es, btw), Put a final and complete end to a certain danger.
Just saying! Bioware will do what they think best, regardless.
As for why the Ultimate Sacrifice, I have only one character, Kaliana Tabris, do it. Reason: She wants to be with Nelaros again. As for the sacrifice, I believe that, when the Archdemon travels thru the taint to possess another body, it tries to possess a body that already has a soul in it. That soul rejects the OG, but the conflict kills the body, which is still attached to the Warden's soul. The Warden's body can not handle this, and dies as a reult of the strain. As the OG is now attached to a body that has again died, it no longer has the strength to travel the taint (the Soul gain such strength from the body), and dies as well.
This is my opinion/belief, believe as you wish.
Nelaros Dies so that Kaliana can live, even as she dies that a world can live. And gains the reward she wishes as a result.