The difference is that the actual stated end goal of the game is to go NG+.Not defeat Aldiun, not battle for New Vegas.
So to use your words, it’s not “Game A plus feature B”, it’s just feature B,
NG+ as a concept stresses immersion, and making it the point of the game shattered it completely. I like the idea or giving an in-game explanation, and the story they used could have worked, but it needed to be a side quest