That's true. Once you get that super-character and the challenge drops to zero it sort of makes the idea of a game pretty pointless.
Have you played a game and tried to unlock every hard-to-get item/character/extras and in the end when you use it/him/her you felt that the "supposed" reward was not worth having because you've already put a lot of time in the game which used to be fun but now bores the hell out of you?
Ok, sorry for the loooong question. My point is, you unlock a very cool powerful character which seems to be pointless because along the way(while en-route to unlocking this character) your other character(s) level up and is almost/more stronger than the newly unlocked character and you're basically finished with the game. Like, there's nothing left for you to do. I mean, it happens a lot not just in RPGs but also in other genre as well.
Just a thought for the day.