This is what they said:
"The second reason is strongly tied to the first; removing this item gives us room to buff champion kits that would otherwise be overpowered if they could use DFG. The most obvious example is Ahri; more 'magey' elements of the character, like her sustained damage or kiting, end up getting washed out because she has so much power to blow up charmed targets with a DFG burst. This ends up forcing us to balance Ahri around her assassin playstyle because there's always the "I can buy DFG" route to consider; removing the item means we can buff her in more interesting ways while still keeping some of her burst because she can't just pick up a DFG and double down on a single playstyle."
They got forced to balance Ahri around her assassin playstyle (when she was supposed to be a mage in a first place) because of the DFG and now that it's gone they can support more her "magey" elements like her sustained damage or kiting like before.
You will still be able to burst people down, because if you hit the charm you're going to land every spell.