Before Nerfing Morgana
Please consider who she's countering first. If a champ has a high pick and win rate, and Morgana has a high pick and win rate too, it's really most likely that she's just a good answer to the problematic champions.
Mid lane, there's Zed running amuck, Sylas, Ahri, Fizz, and maybe a normal mage or two too. Morgana does amazing into assassins, and is a really good counter to Zed especially (telegraphed ult means Q hit, ult on top, also builds zhonyas, and can push hard against) I can go into reasons for the others but similar deal w/ black shield. If you nerf the stuff she's countering (mainly Zed here), then she'll have less of a reason to be picked, or struggle against the stuff coming back.
Support wise, where she is stronger, she doesn't do the best against enchanters, but really shines against a Thresh, Blitz, Pyke, Galio (whos popping up more), and other engage stuff. The strongest one out of the lot here is Thresh, high pick and win rate out of supports, and is always a really good pick. Morg has black shield, and can keep him in place to stop him jumping on an adc.
So yeah, Morgana is high up and strong, but the stuff she counters is also really strong, and she is just a good answer to that. Please don't constantly nerf her while she'll always be good against them. Lissandra is always good against mobile assassins, and is super strong with aftershock, but she just became a boring CC bot with no dmg.
AP Morg mid is actually really fun for me and I'd hate to see it touched because she counters problems in the game at the moment.
Feel free to discuss and everything, this is just my opinion :D It just makes me sad loving to play smth but know it'll face constant nerfs before it feels like crap, while the issues remain untouched, or even buffed.