Back then, Ahri didn't need the 20% damage because she had MUCH higher base damage and scaling. These were lowered to give her the unique Charm effect and make her more interesting and rewarding to play. Now Riot is backpedaling on their own changes, completely flipping on their own design philosophies. Meanwhile, the main issue is that Riot lied to our faces. They promised compensation for removing DFG, but refuse to actually follow up on their own words, or show even the slightest bit of consistency on either past or present matters.
If it were just these changes alone, there would be SIGNIFICANTLY less complaints from the players. However, we were also promised something IN ADDITION and Riot is refusing to let us have it. They lied to our faces.
Summary: Charm has had its mana cost changed to a flat amount at all levels but now increases the magic damage Ahri deals to a Charmed target. Essence Theft now heals her for a specific amount based on the number of enemies she hits with her passive-enhanced spell, as opposed to getting straight spell vamp. Fox Fire now deals less damage when multiple wisps hit the same target. Spirit Rush's base damage and AP ratios have been slightly reduced.
Context: Ahri is a champion with a lot of strengths and few weaknesses, given her high versatility and reliability, so she can consistently apply a lot of pressure on her opponents from relative safety. Ahri’s strengths should be better defined than they currently are. These changes give Ahri players (and her opponents) clear areas of mastery that differentiate the good Ahris from the great Ahris. Our core focuses are:
Make Ahri’s default lane patterns more clear, so both Ahri and her opponents can make plays around them
Make Ahri’s “all-in” gameplay more closely tied with landing Charm to establish Charm as Ahri’s “kiss of death” that needs to be avoided, as well as lower her threat/presence while Charm is on cooldown
Keep Ahri’s core gameplay intact while trimming away strength from the low-gameplay patterns that don’t feel rewarding
http://na.leagueoflegends.com/en/news/game-updates/patch/patch-313-notes