I'll counter that objection. Although I'll agree that Yu Yevon was bad. The Undying wasn't necessary story-wise. It just seemed like a pointless excuse to have a "big" final boss. "Oh let's make him go outside and merge with the Bahamut", I mean it's not original at all is it? It would have been better I you were transported to a place of the Gods (like in Giruvegan or something) and had to fight Venat in possibly some other form (and Vayne would already have died by this point). Besides, we didn't actually get to see what happened to Venat at the end anyway.