In the past riot has said on several occasions that they don't really balance around win rates. They've got other meterics they find more important. They've also said in the past that they've had characters with good win rates who still felt pretty meh to play, I remember them talking about pre-rework Sona like this in particular.
She was really good, but because of her "hidden power" you couldn't easily correlate playing Sona or having Sona on the team with the win. What they did with that information turned into a mess, but it speaks about how a champ can have a good win rate and still not violate the things riot is concerned about. I mean here was a case where you had a good champ but it still felt weak to play.