1 good reason riot should leave janna OP
For all the complaints about Janna's winrate, I think the game is better off with Janna being OP as opposed to someone like Zed, Leblanc, Sej, Lucian, Lee Sin, etc. Basically Janna is the least offensive OP champion imaginable and the game feels better in a Janna meta than when other champ or class is overpowered.
To put it another way, I can't name that many champs I'd want to have a 55% win rate and if Janna is nerfed from the top spot, the odds of someone most of us would not want to see replacing her are quite high.