I agree that the heal nerf was an utterly miserable decision, and that it’s left her in a terrible spot. The statistics all indicate this. The healing needs to be brought back.
Having said that - you probably shouldn’t use gm statistics for one day. At ~2% pick rate, Overbuff records about 100 Mercy games a day. At that rate, you’re expecting huge swings to happen fairly often due to dumb random chance. The weekly averages however are fairly reliable even in GM, and the monthly averages are reliable.
We can currently be very confident that Mercy’s gm win rates are somewhere between 50-51%. That’s still really bad though. The average win rate in GM is 54.4%, so Mercy players are dropping ranks. The healing needs to be brought back, but we shouldn’t be using unreliable numbers to justify it. That way, we won’t have to argue with people on those random rare days when she has a 70% win rate.