In Gameplay Thoughts Aug 29 Meddler admits what I've suspected about balance
Riot wants to make mid lane more exciting for spectators by keeping specific champions strong. They wish they could buff Zed and Yasuo more, but they know it would destroy solo queue.
What he didn't mention is they will not be nerfing LeBlanc anytime soon even though she is probably the most toxic champion to lane against and has a high ban rate. And it goes without saying they won't be nerfing Zed either. Basically in general they don't care if a champ is absurdly popular and/or high ban rate if they deem the champ to be positive for the spectator experience. The spectator experience is taking priority over our gameplay experience.
If you ever really pay attention while watching LCS you might notice that mid lane doesn't get much attention from the observers. It's really boring, even when you get champions like LB in play. Riot's been trying to fix that with the changes to waveclear, cannon minions, and champion balance, etc... But it seems like nothing has worked. Pros still want to play safe and win, so even with high kill pressure champions in mid, they will still do their best to play safe and only go for "free" kills. They will pressure away their lane opponent and secure a big CS lead, which is ofc because the other team is very coordinated and their lane opponent is also a pro.
With the above perspective, in my opinion the attempts to make mid lane more exciting to spectate have all failed while damaging normal play.
I predict that mid lane will never be exciting unless champions who are supposed to be making plays are incentivized to make plays over securing a CS lead. If simply abusing their high kill pressure in lane to secure a CS lead is sufficient to do their job, then those champions will be played as such because it is the safe way to play, especially considering these champs all have the mobility to escape from all-ins and ganks.