On the one hand it would seem very odd to go through the rest of the quest chain for Quel, then take the Unfired Ancient Blade in to level 80 Onyxia to complete The Forging of Quel'Serrar in order to run back to Dire Maul and pick up a 20 level out of date tanking sword, no matter how cool it looks. But since the quest doesn't specify what level she is, it might be possible (It was possible to complete a Naxxramas quest from before TBC launched in the current 25 man Naxxramas). However, unless Ony will continue to drop the sinew, I can't see how folks could possibly complete the epic Hunter quest chain. Does that mean those quests will be gone from the game?
Since I don't play a Hunter and I already have Quel'Serrar, this is all kind of academic to me. But I do wonder: my dearest hope is that they actually update the Quel quests (or make an entirely new one) which allows you to get an updated Quel'Serrar. Perhaps Quel'Zaram? I guess I might as well dream big while I'm at it. Bornakk's post certainly hints very strongly that there will be a successor to Quel dropped from Onyxia.