Ahri is the most popular midlaner in the game, and has over 53% winrate.
Just want to point out that the only midlaners beating her in winrate are those niche, unpopular (but effective in the right hands) picks of Malzahar and Heimerdinger.
Other than that, she's cruising at a constant, reliable 53%+ winrate while also being the most popular midlaner in the entire game, at around 17% pick rate. (Unless you count Annie and Morgana being taken support as competition for her midlane pickrate.)
Most popular, and just about most successful.
And so..................
....................
....they're nerfing Cassiopeia.