Okay, just imagine.
ULTRAXION is the endboss of Cataclysm. SURPRISE mother*kers!
Who the fel he is - might you ask? Fckn nobody! He's a MOST POWERFUL twilight dragon, personal, ultimate creation of Deathwing himself!!! Is that not epic enough for you, ungrateful stupid brats? Yeah, we pulled him out of our asses a minute before you kill him, but just shul up, that's the final boss of expansion.
Deathwing? You say, his story was revealed throughout all the expansion, he was built up to be the boss? Ha ha don't be ridiculous. You just can't challenge him, he's too powerful. He can obliterate you in one move, no way you could fight him in Cata. So he'll not appear in the Dragos Soul raid and just leave back into Deepholm.
We have, in fact, a MUCH BETTER idea. Several expansions later, e.g. in Warlords of Draenor, he will just pop out of nowhere, attack Stromwind and you will fight him. Sounds extremely cool, right?
Now have at thee. "Finally deal with the Legion once and for all". "Hurry and defeat the Fallen Titan". "You must kill KJ and gul'dan before you can face Sargeras". "Take him down at last".
Let's show his true story, make him a shadow behind the scene and...Never apper.
SURPRISE mother*kers! Argus has a world soul too! Say HI to newborn titan and fckn kill it! That's your endboss, a brainless overcharged puppet! That's what you'll sacrifice your artifacts for. That's all you deserve. It's a titan! Why aren't you happy?
The only UNLESS here is if this raid is not the last one. 11 bosses, kinda too few? If Sargeras actually arrives and makes a 3-encounter raid like Deathwing, I'll take all my words back.
But for now - we were just betrayed and spit in the faces. The worst thing they EVER did in this game.