Far from being a cop out or something thrown together at the last minute, this is what the entire series was building up to from the first episode.

Aang was traveling around the world the entire series learning new bending skills.
He mastered all the elements. Other people have mastered single elements and it's like they each had a piece of the puzzle. But you put all the pieces together and sure, he has lots of skills, but they still don't give the big picture. There is still something missing to really explain where these poweres come from or who the heck the Avatar is.

If he was willing to compromise and do what everyone expected him to do he could have just stopped there.
But to really square his powers and duties with his morals he needed to find that last piece, which was spirit bending.

To me, this is what all the mysteries about bending and all his moral conflicts during the whole series were about.