Yeah she's bad if seh's behind but she
^ part of the reason. Which ties in nicely to her post 30 minute stats. Yes, she will do well early, but after 30 minutes she hits a cliff, and that's putting it lightly.
I hope I find your reasoning side, I believe Ahri is one of the most balanced champions in the game.
As an assassin, she has an "I win condition" in her charm. Yes she has good wave clear and pick potential, her damage is alright compared to other assassins, but she is not without weaknesses.
First off, her 1-5 is very vulnerable should she ever go for a trade with E she leaves herself susceptible to getting ganked.
Her range is limited, especially with her W. She is not Xerath. Her R is a great escape/engage/pick tool, but when it's down she is very vulnerable getting her to use it and repeat ganking brings her back to 1-5 status. Team fights one of Ahris biggest weaknesses as shes really not able to contribute much if a tank is in front of her. Ahri does not posses the sustainable damage to deal with a on par tank. Thus why she loses after 30 minutes so much more.
She is an early/midgame champion with an obvious late game trade off.
I don't want to go too much into detail on this as I don't want it to be huge, but one more thing I'll touch on is the power of defensive AP items against her. Banshees and Zhonyas ruin Ahris day. As I said she does not have sustained DPS. If Ahri were to miss or get blocked, she is fairly useless for the next 8 seconds.