Corki PBE Nerfs are too severe

With the issues with Camille, Rengar, and leathality right now, why is Corki getting nerfed? While Corki is a AD caster, he doesnt use leathality. Mid he has a win rate of 53% and is 10th in win rate. Bot Corki is at 50% and is #10 in win rate. To me when i see corki's stats on champion.gg I see a mostly balanced champ.

IMO even a slight nerf will put corki into the 20s in win rate. Worst part is when i look at the context link on the PBE, and i go thru Medler's posts i do not see any reasoning for the Corki nerfs.

Here is the nerf in question:

Missile Barrage (R):
    Base damage lowered from 100/130/160 to 60/80/100.
    AP scaling on missiles lowered from 30% to 18%
    Total AD scaling on missiles lowered from 20/60/100% to 10/35/60%.
    Big One damage increased from 50% to 150% increased damage.

I mean why is Corki getting a 50% damage nerf on his Ult? Missile Barrage is a ultimate skill, these skills are suppose to be strong. This gutting makes Corki's ult just another useless skill that he uses repeatedly to clear waves. IMO Corki using R to clear waves is the behavior RIot should be trying to STOP. I can only guess this is the behavior issue Riot has, since i can not see any reason why Corki should be nerfed. For example I have never seen any LCS games where Corki snowballed into a overpowered state, and carried the game.

IMO if riot really wanted to fix the using R to wave clear behavior, they should just raise his mana costs on r. The fact they didnt first touch his mana costs shows who ever did this doesent really play Corki enough.

IMO this is a gutting and a reprisal. I bet some rioter fought a corki and got poked to hell because he never bought mr and decided "this is too stronk"..... Corki does mostly magic damage, thats why he is a ADC that wears sorc shoes.